Two level memory full line writes

ABSTRACT

A memory controller receives a memory invalidation request that references a line of far memory in a two level system memory topology with far memory and near memory, identifies an address of the near memory corresponding to the line, and reads data at the address to determine whether a copy of the line is in the near memory. Data of the address is to be flushed to the far memory if the data includes a copy of another line of the far memory and the copy of the other line is dirty. A completion is sent for the memory invalidation request to indicate that a coherence agent is granted exclusive access to the line. With exclusive access, the line is to be modified to generate a modified version of the line and the address of the near memory is to be overwritten with the modified version of the line.

CROSS REFERENCE TO RELATED APPLICATIONS

This Application is a continuation (and claims the benefit of priorityunder 35 U.S.C. §120) of U.S. application Ser. No. 14/670,857, filedMar. 27, 2015 and entitled TWO LEVEL MEMORY FULL LINE WRITES. Thedisclosure of the prior Application is considered part of and isincorporated by reference in the disclosure of this Application.

FIELD

The present disclosure relates in general to the field of computingarchitectures, and more specifically, to buffered memory protocols.

BACKGROUND

Advances in semi-conductor processing and logic design have permitted anincrease in the amount of logic that may be present on integratedcircuit devices. As a corollary, computer system configurations haveevolved from a single or multiple integrated circuits in a system tomultiple cores, multiple hardware threads, and multiple logicalprocessors present on individual integrated circuits, as well as otherinterfaces integrated within such processors. A processor or integratedcircuit typically comprises a single physical processor die, where theprocessor die may include any number of cores, hardware threads, logicalprocessors, interfaces, memory, controller hubs, etc.

As a result of the greater ability to fit more processing power insmaller packages, smaller computing devices have increased inpopularity. Smartphones, tablets, ultrathin notebooks, and other userequipment have grown exponentially. However, these smaller devices arereliant on servers both for data storage and complex processing thatexceeds the form factor. Consequently, the demand in thehigh-performance computing market (i.e. server space) has alsoincreased. For instance, in modern servers, there is typically not onlya single processor with multiple cores, but also multiple physicalprocessors (also referred to as multiple sockets) to increase thecomputing power. But as the processing power grows along with the numberof devices in a computing system, the communication between sockets andother devices becomes more critical.

In fact, interconnects have grown from more traditional multi-drop busesthat primarily handled electrical communications to full blowninterconnect architectures that facilitate fast communication.Unfortunately, as the demand for future processors to consume at evenhigher-rates corresponding demand is placed on the capabilities ofexisting interconnect architectures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a simplified block diagram of a system including aserial point-to-point interconnect to connect I/O devices in a computersystem in accordance with one embodiment;

FIG. 2 illustrates a simplified block diagram of a layered protocolstack in accordance with one embodiment;

FIG. 3 illustrates an embodiment of a transaction descriptor.

FIG. 4 illustrates an embodiment of a serial point-to-point link.

FIG. 5 illustrates embodiments of potential High PerformanceInterconnect (HPI) system configurations.

FIG. 6 illustrates an embodiment of a layered protocol stack associatedwith HPI.

FIG. 7 illustrates a representation of an example state machine.

FIG. 8 illustrates a representation of an example flit sent over anexample twenty-lane data link.

FIG. 9 illustrates a representation of an example flit sent over anexample eight-lane data link.

FIG. 10 illustrates a representation of an example multi-slot flit.

FIG. 11 illustrates a representation of an example flit sent over anexample eight-lane data link.

FIG. 12 illustrates a representation of an example multi-slot flitincluding a debug message.

FIG. 13 illustrates a representation of a system utilizing a bufferedmemory protocol.

FIG. 14 illustrates a representation of a buffer device supporting abuffered memory protocol.

FIGS. 15A-15C illustrate representations of example flits in a bufferedmemory protocol.

FIG. 16 illustrates an example transaction involving two level memory,in accordance with some implementations.

FIGS. 17A-17B illustrates example transactions involving two levelmemory, in accordance with some implementations.

FIG. 18 illustrates an embodiment of a block for a computing systemincluding multiple processor sockets.

FIG. 19 illustrates another embodiment of a block diagram for acomputing system including a multicore processor.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth,such as examples of specific types of processors and systemconfigurations, specific hardware structures, specific architectural andmicro architectural details, specific register configurations, specificinstruction types, specific system components, specific processorpipeline stages, specific interconnect layers, specificpacket/transaction configurations, specific transaction names, specificprotocol exchanges, specific link widths, specific implementations, andoperation etc. in order to provide a thorough understanding of thepresent invention. It may be apparent, however, to one skilled in theart that these specific details need not necessarily be employed topractice the subject matter of the present disclosure. In otherinstances, well detailed description of known components or methods hasbeen avoided, such as specific and alternative processor architectures,specific logic circuits/code for described algorithms, specific firmwarecode, low-level interconnect operation, specific logic configurations,specific manufacturing techniques and materials, specific compilerimplementations, specific expression of algorithms in code, specificpower down and gating techniques/logic and other specific operationaldetails of computer system in order to avoid unnecessarily obscuring thepresent disclosure.

Although the following embodiments may be described with reference toenergy conservation, energy efficiency, processing efficiency, and so onin specific integrated circuits, such as in computing platforms ormicroprocessors, other embodiments are applicable to other types ofintegrated circuits and logic devices. Similar techniques and teachingsof embodiments described herein may be applied to other types ofcircuits or semiconductor devices that may also benefit from suchfeatures. For example, the disclosed embodiments are not limited toserver computer system, desktop computer systems, laptops, Ultrabooks™,but may be also used in other devices, such as handheld devices,smartphones, tablets, other thin notebooks, systems on a chip (SOC)devices, and embedded applications. Some examples of handheld devicesinclude cellular phones, Internet protocol devices, digital cameras,personal digital assistants (PDAs), and handheld PCs. Here, similartechniques for a high-performance interconnect may be applied toincrease performance (or even save power) in a low power interconnect.Embedded applications typically include a microcontroller, a digitalsignal processor (DSP), a system on a chip, network computers (NetPC),set-top boxes, network hubs, wide area network (WAN) switches, or anyother system that can perform the functions and operations taught below.Moreover, the apparatus', methods, and systems described herein are notlimited to physical computing devices, but may also relate to softwareoptimizations for energy conservation and efficiency. As may becomereadily apparent in the description below, the embodiments of methods,apparatus', and systems described herein (whether in reference tohardware, firmware, software, or a combination thereof) may beconsidered vital to a “green technology” future balanced withperformance considerations.

As computing systems are advancing, the components therein are becomingmore complex. The interconnect architecture to couple and communicatebetween the components has also increased in complexity to ensurebandwidth demand is met for optimal component operation. Furthermore,different market segments demand different aspects of interconnectarchitectures to suit the respective market. For example, serversrequire higher performance, while the mobile ecosystem is sometimes ableto sacrifice overall performance for power savings. Yet, it is asingular purpose of most fabrics to provide highest possible performancewith maximum power saving. Further, a variety of different interconnectscan potentially benefit from subject matter described herein.

The Peripheral Component Interconnect (PCI) Express (PCIe) interconnectfabric architecture and QuickPath Interconnect (QPI) fabricarchitecture, among other examples, can potentially be improvedaccording to one or more principles described herein, among otherexamples. For instance, a primary goal of PCIe is to enable componentsand devices from different vendors to inter-operate in an openarchitecture, spanning multiple market segments; Clients (Desktops andMobile), Servers (Standard and Enterprise), and Embedded andCommunication devices. PCI Express is a high performance, generalpurpose I/O interconnect defined for a wide variety of future computingand communication platforms. Some PCI attributes, such as its usagemodel, load-store architecture, and software interfaces, have beenmaintained through its revisions, whereas previous parallel busimplementations have been replaced by a highly scalable, fully serialinterface. The more recent versions of PCI Express take advantage ofadvances in point-to-point interconnects, Switch-based technology, andpacketized protocol to deliver new levels of performance and features.Power Management, Quality Of Service (QoS), Hot-Plug/Hot-Swap support,Data Integrity, and Error Handling are among some of the advancedfeatures supported by PCI Express. Although the primary discussionherein is in reference to a new high-performance interconnect (HPI)architecture, aspects of the invention described herein may be appliedto other interconnect architectures, such as a PCIe-compliantarchitecture, a QPI-compliant architecture, a MIPI compliantarchitecture, a high-performance architecture, or other knowninterconnect architecture.

Referring to FIG. 1, an embodiment of a fabric composed ofpoint-to-point Links that interconnect a set of components isillustrated. System 100 includes processor 105 and system memory 110coupled to controller hub 115. Processor 105 can include any processingelement, such as a microprocessor, a host processor, an embeddedprocessor, a co-processor, or other processor. Processor 105 is coupledto controller hub 115 through front-side bus (FSB) 106. In oneembodiment, FSB 106 is a serial point-to-point interconnect as describedbelow. In another embodiment, link 106 includes a serial, differentialinterconnect architecture that is compliant with different interconnectstandard.

System memory 110 includes any memory device, such as random accessmemory (RAM), non-volatile (NV) memory, or other memory accessible bydevices in system 100. System memory 110 is coupled to controller hub115 through memory interface 116. Examples of a memory interface includea double-data rate (DDR) memory interface, a dual-channel DDR memoryinterface, and a dynamic RAM (DRAM) memory interface.

In one embodiment, controller hub 115 can include a root hub, rootcomplex, or root controller, such as in a PCIe interconnectionhierarchy. Examples of controller hub 115 include a chipset, a memorycontroller hub (MCH), a northbridge, an interconnect controller hub(ICH) a southbridge, and a root controller/hub. Often the term chipsetrefers to two physically separate controller hubs, e.g., a memorycontroller hub (MCH) coupled to an interconnect controller hub (ICH).Note that current systems often include the MCH integrated withprocessor 105, while controller 115 is to communicate with I/O devices,in a similar manner as described below. In some embodiments,peer-to-peer routing is optionally supported through root complex 115.

Here, controller hub 115 is coupled to switch/bridge 120 through seriallink 119. Input/output modules 117 and 121, which may also be referredto as interfaces/ports 117 and 121, can include/implement a layeredprotocol stack to provide communication between controller hub 115 andswitch 120. In one embodiment, multiple devices are capable of beingcoupled to switch 120.

Switch/bridge 120 routes packets/messages from device 125 upstream, i.e.up a hierarchy towards a root complex, to controller hub 115 anddownstream, i.e. down a hierarchy away from a root controller, fromprocessor 105 or system memory 110 to device 125. Switch 120, in oneembodiment, is referred to as a logical assembly of multiple virtualPCI-to-PCI bridge devices. Device 125 includes any internal or externaldevice or component to be coupled to an electronic system, such as anI/O device, a Network Interface Controller (NIC), an add-in card, anaudio processor, a network processor, a hard-drive, a storage device, aCD/DVD ROM, a monitor, a printer, a mouse, a keyboard, a router, aportable storage device, a Firewire device, a Universal Serial Bus (USB)device, a scanner, and other input/output devices. Often in the PCIevernacular, such as device, is referred to as an endpoint. Although notspecifically shown, device 125 may include a bridge (e.g., a PCIe toPCI/PCI-X bridge) to support legacy or other versions of devices orinterconnect fabrics supported by such devices.

Graphics accelerator 130 can also be coupled to controller hub 115through serial link 132. In one embodiment, graphics accelerator 130 iscoupled to an MCH, which is coupled to an ICH. Switch 120, andaccordingly I/O device 125, is then coupled to the ICH. I/O modules 131and 118 are also to implement a layered protocol stack to communicatebetween graphics accelerator 130 and controller hub 115. Similar to theMCH discussion above, a graphics controller or the graphics accelerator130 itself may be integrated in processor 105.

Turning to FIG. 2 an embodiment of a layered protocol stack isillustrated. Layered protocol stack 200 can includes any form of alayered communication stack, such as a QPI stack, a PCIe stack, a nextgeneration high performance computing interconnect (HPI) stack, or otherlayered stack. In one embodiment, protocol stack 200 can includetransaction layer 205, link layer 210, and physical layer 220. Aninterface, such as interfaces 117, 118, 121, 122, 126, and 131 in FIG.1, may be represented as communication protocol stack 200.Representation as a communication protocol stack may also be referred toas a module or interface implementing/including a protocol stack.

Packets can be used to communicate information between components.Packets can be formed in the Transaction Layer 205 and Data Link Layer210 to carry the information from the transmitting component to thereceiving component. As the transmitted packets flow through the otherlayers, they are extended with additional information used to handlepackets at those layers. At the receiving side the reverse processoccurs and packets get transformed from their Physical Layer 220representation to the Data Link Layer 210 representation and finally(for Transaction Layer Packets) to the form that can be processed by theTransaction Layer 205 of the receiving device.

In one embodiment, transaction layer 205 can provide an interfacebetween a device's processing core and the interconnect architecture,such as Data Link Layer 210 and Physical Layer 220. In this regard, aprimary responsibility of the transaction layer 205 can include theassembly and disassembly of packets (i.e., transaction layer packets, orTLPs). The translation layer 205 can also manage credit-based flowcontrol for TLPs. In some implementations, split transactions can beutilized, i.e., transactions with request and response separated bytime, allowing a link to carry other traffic while the target devicegathers data for the response, among other examples.

Credit-based flow control can be used to realize virtual channels andnetworks utilizing the interconnect fabric. In one example, a device canadvertise an initial amount of credits for each of the receive buffersin Transaction Layer 205. An external device at the opposite end of thelink, such as controller hub 115 in FIG. 1, can count the number ofcredits consumed by each TLP. A transaction may be transmitted if thetransaction does not exceed a credit limit. Upon receiving a response anamount of credit is restored. One example of an advantage of such acredit scheme is that the latency of credit return does not affectperformance, provided that the credit limit is not encountered, amongother potential advantages.

In one embodiment, four transaction address spaces can include aconfiguration address space, a memory address space, an input/outputaddress space, and a message address space. Memory space transactionsinclude one or more of read requests and write requests to transfer datato/from a memory-mapped location. In one embodiment, memory spacetransactions are capable of using two different address formats, e.g., ashort address format, such as a 32-bit address, or a long addressformat, such as 64-bit address. Configuration space transactions can beused to access configuration space of various devices connected to theinterconnect. Transactions to the configuration space can include readrequests and write requests. Message space transactions (or, simplymessages) can also be defined to support in-band communication betweeninterconnect agents. Therefore, in one example embodiment, transactionlayer 205 can assemble packet header/payload 206.

Quickly referring to FIG. 3, an example embodiment of a transactionlayer packet descriptor is illustrated. In one embodiment, transactiondescriptor 300 can be a mechanism for carrying transaction information.In this regard, transaction descriptor 300 supports identification oftransactions in a system. Other potential uses include trackingmodifications of default transaction ordering and association oftransaction with channels. For instance, transaction descriptor 300 caninclude global identifier field 302, attributes field 304 and channelidentifier field 306. In the illustrated example, global identifierfield 302 is depicted comprising local transaction identifier field 308and source identifier field 310. In one embodiment, global transactionidentifier 302 is unique for all outstanding requests.

According to one implementation, local transaction identifier field 308is a field generated by a requesting agent, and can be unique for alloutstanding requests that require a completion for that requestingagent. Furthermore, in this example, source identifier 310 uniquelyidentifies the requestor agent within an interconnect hierarchy.Accordingly, together with source ID 310, local transaction identifier308 field provides global identification of a transaction within ahierarchy domain.

Attributes field 304 specifies characteristics and relationships of thetransaction. In this regard, attributes field 304 is potentially used toprovide additional information that allows modification of the defaulthandling of transactions. In one embodiment, attributes field 304includes priority field 312, reserved field 314, ordering field 316, andno-snoop field 318. Here, priority sub-field 312 may be modified by aninitiator to assign a priority to the transaction. Reserved attributefield 314 is left reserved for future, or vendor-defined usage. Possibleusage models using priority or security attributes may be implementedusing the reserved attribute field.

In this example, ordering attribute field 316 is used to supply optionalinformation conveying the type of ordering that may modify defaultordering rules. According to one example implementation, an orderingattribute of “0” denotes default ordering rules are to apply, wherein anordering attribute of “1” denotes relaxed ordering, wherein writes canpass writes in the same direction, and read completions can pass writesin the same direction. Snoop attribute field 318 is utilized todetermine if transactions are snooped. As shown, channel ID Field 306identifies a channel that a transaction is associated with.

Returning to the discussion of FIG. 2, a Link layer 210, also referredto as data link layer 210, can act as an intermediate stage betweentransaction layer 205 and the physical layer 220. In one embodiment, aresponsibility of the data link layer 210 is providing a reliablemechanism for exchanging Transaction Layer Packets (TLPs) between twocomponents on a link. One side of the Data Link Layer 210 accepts TLPsassembled by the Transaction Layer 205, applies packet sequenceidentifier 211, i.e. an identification number or packet number,calculates and applies an error detection code, i.e. CRC 212, andsubmits the modified TLPs to the Physical Layer 220 for transmissionacross a physical to an external device.

In one example, physical layer 220 includes logical sub block 221 andelectrical sub-block 222 to physically transmit a packet to an externaldevice. Here, logical sub-block 221 is responsible for the “digital”functions of Physical Layer 221. In this regard, the logical sub-blockcan include a transmit section to prepare outgoing information fortransmission by physical sub-block 222, and a receiver section toidentify and prepare received information before passing it to the LinkLayer 210.

Physical block 222 includes a transmitter and a receiver. Thetransmitter is supplied by logical sub-block 221 with symbols, which thetransmitter serializes and transmits onto to an external device. Thereceiver is supplied with serialized symbols from an external device andtransforms the received signals into a bit-stream. The bit-stream isde-serialized and supplied to logical sub-block 221. In one exampleembodiment, an 8b/10b transmission code is employed, where ten-bitsymbols are transmitted/received. Here, special symbols are used toframe a packet with frames 223. In addition, in one example, thereceiver also provides a symbol clock recovered from the incoming serialstream.

As stated above, although transaction layer 205, link layer 210, andphysical layer 220 are discussed in reference to a specific embodimentof a protocol stack (such as a PCIe protocol stack), a layered protocolstack is not so limited. In fact, any layered protocol may beincluded/implemented and adopt features discussed herein. As an example,a port/interface that is represented as a layered protocol can include:(1) a first layer to assemble packets, i.e. a transaction layer; asecond layer to sequence packets, i.e. a link layer; and a third layerto transmit the packets, i.e. a physical layer. As a specific example, ahigh performance interconnect layered protocol, as described herein, isutilized.

Referring next to FIG. 4, an example embodiment of a serial point topoint fabric is illustrated. A serial point-to-point link can includeany transmission path for transmitting serial data. In the embodimentshown, a link can include two, low-voltage, differentially driven signalpairs: a transmit pair 406/411 and a receive pair 412/407. Accordingly,device 405 includes transmission logic 406 to transmit data to device410 and receiving logic 407 to receive data from device 410. In otherwords, two transmitting paths, i.e. paths 416 and 417, and two receivingpaths, i.e. paths 418 and 419, are included in some implementations of alink.

A transmission path refers to any path for transmitting data, such as atransmission line, a copper line, an optical line, a wirelesscommunication channel, an infrared communication link, or othercommunication path. A connection between two devices, such as device 405and device 410, is referred to as a link, such as link 415. A link maysupport one lane—each lane representing a set of differential signalpairs (one pair for transmission, one pair for reception). To scalebandwidth, a link may aggregate multiple lanes denoted by xN, where N isany supported link width, such as 1, 2, 4, 8, 12, 16, 32, 64, or wider.

A differential pair can refer to two transmission paths, such as lines416 and 417, to transmit differential signals. As an example, when line416 toggles from a low voltage level to a high voltage level, i.e. arising edge, line 417 drives from a high logic level to a low logiclevel, i.e. a falling edge. Differential signals potentially demonstratebetter electrical characteristics, such as better signal integrity, i.e.cross-coupling, voltage overshoot/undershoot, ringing, among otherexample advantages. This allows for a better timing window, whichenables faster transmission frequencies.

In one embodiment, a new High Performance Interconnect (HPI) isprovided. HPI can include a next-generation cache-coherent, link-basedinterconnect. As one example, HPI may be utilized in high performancecomputing platforms, such as workstations or servers, including insystems where PCIe or another interconnect protocol is typically used toconnect processors, accelerators, I/O devices, and the like. However,HPI is not so limited. Instead, HPI may be utilized in any of thesystems or platforms described herein. Furthermore, the individual ideasdeveloped may be applied to other interconnects and platforms, such asPCIe, MIPI, QPI, etc.

To support multiple devices, in one example implementation, HPI caninclude an Instruction Set Architecture (ISA) agnostic (i.e. HPI is ableto be implemented in multiple different devices). In another scenario,HPI may also be utilized to connect high performance I/O devices, notjust processors or accelerators. For example, a high performance PCIedevice may be coupled to HPI through an appropriate translation bridge(i.e. HPI to PCIe). Moreover, the HPI links may be utilized by many HPIbased devices, such as processors, in various ways (e.g. stars, rings,meshes, etc.). FIG. 5 illustrates example implementations of multiplepotential multi-socket configurations. A two-socket configuration 505,as depicted, can include two HPI links; however, in otherimplementations, one HPI link may be utilized. For larger topologies,any configuration may be utilized as long as an identifier (ID) isassignable and there is some form of virtual path, among otheradditional or substitute features. As shown, in one example, a foursocket configuration 510 has an HPI link from each processor to another.But in the eight socket implementation shown in configuration 515, notevery socket is directly connected to each other through an HPI link.However, if a virtual path or channel exists between the processors, theconfiguration is supported. A range of supported processors includes2-32 in a native domain. Higher numbers of processors may be reachedthrough use of multiple domains or other interconnects between nodecontrollers, among other examples.

The HPI architecture includes a definition of a layered protocolarchitecture, including in some examples, protocol layers (coherent,non-coherent, and, optionally, other memory based protocols), a routinglayer, a link layer, and a physical layer. Furthermore, HPI can furtherinclude enhancements related to power managers (such as power controlunits (PCUs)), design for test and debug (DFT), fault handling,registers, security, among other examples. FIG. 5 illustrates anembodiment of an example HPI layered protocol stack. In someimplementations, at least some of the layers illustrated in FIG. 5 maybe optional. Each layer deals with its own level of granularity orquantum of information (the protocol layer 605 a,b with packets 630,link layer 610 a,b with flits 635, and physical layer 605 a,b with phits640). Note that a packet, in some embodiments, may include partialflits, a single flit, or multiple flits based on the implementation.

As a first example, a width of a phit 640 includes a 1 to 1 mapping oflink width to bits (e.g. 20 bit link width includes a phit of 20 bits,etc.). Flits may have a greater size, such as 184, 192, or 200 bits.Note that if phit 640 is 20 bits wide and the size of flit 635 is 184bits then it takes a fractional number of phits 640 to transmit one flit635 (e.g. 9.2 phits at 20 bits to transmit an 184 bit flit 635 or 9.6 at20 bits to transmit a 192 bit flit, among other examples). Note thatwidths of the fundamental link at the physical layer may vary. Forexample, the number of lanes per direction may include 2, 4, 6, 8, 10,12, 14, 16, 18, 20, 22, 24, etc. In one embodiment, link layer 610 a,bis capable of embedding multiple pieces of different transactions in asingle flit, and one or multiple headers (e.g. 1, 2, 3, 4) may beembedded within the flit. In one example, HPI splits the headers intocorresponding slots to enable multiple messages in the flit destined fordifferent nodes.

Physical layer 605 a,b, in one embodiment, can be responsible for thefast transfer of information on the physical medium (electrical oroptical etc.). The physical link can be point-to-point between two Linklayer entities, such as layer 605 a and 605 b. The Link layer 610 a,bcan abstract the Physical layer 605 a,b from the upper layers andprovides the capability to reliably transfer data (as well as requests)and manage flow control between two directly connected entities. TheLink Layer can also be responsible for virtualizing the physical channelinto multiple virtual channels and message classes. The Protocol layer620 a,b relies on the Link layer 610 a,b to map protocol messages intothe appropriate message classes and virtual channels before handing themto the Physical layer 605 a,b for transfer across the physical links.Link layer 610 a,b may support multiple messages, such as a request,snoop, response, writeback, non-coherent data, among other examples.

The Physical layer 605 a,b (or PHY) of HPI can be implemented above theelectrical layer (i.e. electrical conductors connecting two components)and below the link layer 610 a,b, as illustrated in FIG. 6. The Physicallayer and corresponding logic can reside on each agent and connects thelink layers on two agents (A and B) separated from each other (e.g. ondevices on either side of a link). The local and remote electricallayers are connected by physical media (e.g. wires, conductors, optical,etc.). The Physical layer 605 a,b, in one embodiment, has two majorphases, initialization and operation. During initialization, theconnection is opaque to the link layer and signaling may involve acombination of timed states and handshake events. During operation, theconnection is transparent to the link layer and signaling is at a speed,with all lanes operating together as a single link. During the operationphase, the Physical layer transports flits from agent A to agent B andfrom agent B to agent A. The connection is also referred to as a linkand abstracts some physical aspects including media, width and speedfrom the link layers while exchanging flits and control/status ofcurrent configuration (e.g. width) with the link layer. Theinitialization phase includes minor phases e.g. Polling, Configuration.The operation phase also includes minor phases (e.g. link powermanagement states).

In one embodiment, Link layer 610 a,b can be implemented so as toprovide reliable data transfer between two protocol or routing entities.The Link layer can abstract Physical layer 605 a,b from the Protocollayer 620 a,b, and can be responsible for the flow control between twoprotocol agents (A, B), and provide virtual channel services to theProtocol layer (Message Classes) and Routing layer (Virtual Networks).The interface between the Protocol layer 620 a,b and the Link Layer 610a,b can typically be at the packet level. In one embodiment, thesmallest transfer unit at the Link Layer is referred to as a flit whicha specified number of bits, such as 192 bits or some other denomination.The Link Layer 610 a,b relies on the Physical layer 605 a,b to frame thePhysical layer's 605 a,b unit of transfer (phit) into the Link Layer's610 a,b unit of transfer (flit). In addition, the Link Layer 610 a,b maybe logically broken into two parts, a sender and a receiver. Asender/receiver pair on one entity may be connected to a receiver/senderpair on another entity. Flow Control is often performed on both a flitand a packet basis. Error detection and correction is also potentiallyperformed on a flit level basis.

In one embodiment, Routing layer 615 a,b can provide a flexible anddistributed method to route HPI transactions from a source to adestination. The scheme is flexible since routing algorithms formultiple topologies may be specified through programmable routing tablesat each router (the programming in one embodiment is performed byfirmware, software, or a combination thereof). The routing functionalitymay be distributed; the routing may be done through a series of routingsteps, with each routing step being defined through a lookup of a tableat either the source, intermediate, or destination routers. The lookupat a source may be used to inject a HPI packet into the HPI fabric. Thelookup at an intermediate router may be used to route an HPI packet froman input port to an output port. The lookup at a destination port may beused to target the destination HPI protocol agent. Note that the Routinglayer, in some implementations, can be thin since the routing tables,and, hence the routing algorithms, are not specifically defined byspecification. This allows for flexibility and a variety of usagemodels, including flexible platform architectural topologies to bedefined by the system implementation. The Routing layer 615 a,b relieson the Link layer 610 a,b for providing the use of up to three (or more)virtual networks (VNs)—in one example, two deadlock-free VNs, VN0 andVN1 with several message classes defined in each virtual network. Ashared adaptive virtual network (VNA) may be defined in the Link layer,but this adaptive network may not be exposed directly in routingconcepts, since each message class and virtual network may havededicated resources and guaranteed forward progress, among otherfeatures and examples.

In one embodiment, HPI can include a Coherence Protocol layer 620 a,b tosupport agents caching lines of data from memory. An agent wishing tocache memory data may use the coherence protocol to read the line ofdata to load into its cache. An agent wishing to modify a line of datain its cache may use the coherence protocol to acquire ownership of theline before modifying the data. After modifying a line, an agent mayfollow protocol requirements of keeping it in its cache until it eitherwrites the line back to memory or includes the line in a response to anexternal request. Lastly, an agent may fulfill external requests toinvalidate a line in its cache. The protocol ensures coherency of thedata by dictating the rules all caching agents (also referred to as“coherence agents”) may follow. It also provides the means for agentswithout caches to coherently read and write memory data.

Two conditions may be enforced to support transactions utilizing the HPICoherence Protocol. First, the protocol can maintain data consistency,as an example, on a per-address basis, among data in agents' caches andbetween those data and the data in memory. Informally, data consistencymay refer to each valid line of data in an agent's cache representing amost up-to-date value of the data and data transmitted in a coherenceprotocol packet can represent the most up-to-date value of the data atthe time it was sent. When no valid copy of the data exists in caches orin transmission, the protocol may ensure the most up-to-date value ofthe data resides in memory. Second, the protocol can providewell-defined commitment points for requests. Commitment points for readsmay indicate when the data is usable; and for writes they may indicatewhen the written data is globally observable and will be loaded bysubsequent reads. The protocol may support these commitment points forboth cacheable and uncacheable (UC) requests in the coherent memoryspace.

In some implementations, HPI can utilize an embedded clock. A clocksignal can be embedded in data transmitted using the interconnect. Withthe clock signal embedded in the data, distinct and dedicated clocklanes can be omitted. This can be useful, for instance, as it can allowmore pins of a device to be dedicated to data transfer, particularly insystems where space for pins is at a premium.

A link can be established between two agents on either side of aninterconnect. An agent sending data can be a local agent and the agentreceiving the data can be a remote agent. State machines can be employedby both agents to manage various aspects of the link. In one embodiment,the Physical layer data path can transmit flits from the link layer tothe electrical front-end. The control path, in one implementation,includes a state machine (also referred to as a link training statemachine or the similar). The state machine's actions and exits fromstates may depend on internal signals, timers, external signals or otherinformation. In fact, some of the states, such as a few initializationstates, may have timers to provide a timeout value to exit a state. Notethat detect, in some embodiments, refers to detecting an event on bothlegs of a lane; but not necessarily simultaneously. However, in otherembodiments, detect refers to detection of an event by an agent ofreference. Debounce, as one example, refers to sustained assertion of asignal. In one embodiment, HPI supports operation in the event ofnon-function lanes. Here, lanes may be dropped at specific states.

States defined in the state machine can include reset states,initialization states, and operational states, among other categoriesand subcategories. In one example, some initialization states can have asecondary timer which is used to exit the state on a timeout(essentially an abort due to failure to make progress in the state). Anabort may include updating of registers, such as status register. Somestates can also have primary timer(s) which are used to time the primaryfunctions in the state. Other states can be defined such that internalor external signals (such as handshake protocols) drive transition fromthe state to another state, among other examples.

A state machine may also support debug through single step, freeze oninitialization abort and use of testers. Here, state exits can bepostponed/held until the debug software is ready. In some instance, theexit can be postponed/held until the secondary timeout. Actions andexits, in one embodiment, can be based on exchange of trainingsequences. In one embodiment, the link state machine is to run in thelocal agent clock domain and transition from one state to the next is tocoincide with a transmitter training sequence boundary. Status registersmay be utilized to reflect the current state.

FIG. 7 illustrates a representation of at least a portion of a statemachine used by agents in one example implementation of HPI. It shouldbe appreciated that the states included in the state table of FIG. 7include a non-exhaustive listing of possible states. For instance, sometransitions are omitted to simplify the diagram. Also, some states maybe combined, split, or omitted, while others might be added. Such statescan include:

Event reset state: entered on a warm or cold reset event. Restoresdefault values. Initialize counters (e.g., sync counters). May exit toanother state, such as another reset state.

Timed reset state: timed state for in-band reset. May drive a predefinedelectrical ordered set (EOS) so remote receivers are capable ofdetecting the EOS and entering the timed reset as well. Receiver haslanes holding electrical settings. May exit to an agent to calibratereset state.

Calibrate reset state: calibration without signaling on the lane (e.g.receiver calibration state) or turning drivers off. May be apredetermined amount of time in the state based on a timer. May set anoperational speed. May act as a wait state when a port is not enabled.May include minimum residency time. Receiver conditioning or staggeringoff may occur based on design. May exit to a receiver detect state aftera timeout and/or completion of calibration.

Receiver detect state: detect presence of a receiver on lane(s). Maylook for receiver termination (e.g. receiver pulldown insertion). Mayexit to calibrate reset state upon a specified value being set or whenanother specified value is not set. May exit to transmitter calibratestate if a receiver is detected or a timeout is reached.

Transmitter calibrate state: for transmitter calibrations. May be atimed state allocated for transmitter calibrations. May includesignaling on a lane. May continuously drive an EOS, such as an electricidle exit ordered set (or EIEIOS). May exit to compliance state whendone calibrating or on expiration of a timer. May exit to transmitterdetect state if a counter has expired or a secondary timeout hasoccurred.

Transmitter detect state: qualifies valid signaling. May be a handshakestate where an agent completes actions and exits to a next state basedon remote agent signaling. Receiver may qualify valid signaling fromtransmitter. Receiver, in one embodiment, looks for a wake detect, andif debounced on one or more lanes looks for it on the other lanes.Transmitter drives a detect signal. May exit to a polling state inresponse to debounce being completed for all lanes and/or a timeout orif debounce on all lanes is not complete and there is a timeout. Here,one or more monitor lanes may be kept awake to debounce a wake signal.And if debounced then the other lanes are potentially debounced. Thiscan enable power savings in low power states.

Polling state: receiver adapts, locks on bits (e.g., by initializing adrift buffer), locks on bytes (e.g., by identifying symbol boundaries),and locks on training sequence boundaries (e.g., by locking to anelectrical ordered set (EOS) and/or a training sequence header). Lanesmay then be deskewed. Handshakes can also be completed to exit pollingto one of several potential states. For instance, an exit can be caused(by an agent) to any one of a Link Width state, Compliance state,Loopback Marker state, or Reset state. Handshaking can include thecompletion of various defined receiver actions or conditions and thesending of an acknowledge message (ACK) to trigger the exit to the statecorresponding to the completed set of actions and the ACK. For pollingdeskew, lane to lane skew at remote transmitter may be capped at a firstlength for top speed and a second length for slow speed. Deskew may beperformed in a slow mode as well as an operational mode. Receiver mayhave a specific maximum to deskew lane-to-lane skew, such as 8, 16, or32 intervals of skew. Receiver actions may also include latency fixingin some implementations. Receiver actions, in one embodiment, can becompleted on successful deskew of a valid lane map. A successfulhandshake can be achieved, in one example, when a number of consecutivetraining sequence headers are received with acknowledgements and anumber of training sequences with an acknowledge are transmitted afterthe receiver has completed its actions.

Compliance state: entered from the Polling state. An agent may be made aCompliance master or slave for validation purposes. The device (actingas master) can send a compliance pattern to another device (slave) andthe slave can loop back the pattern after re-timing it to its localclock (e.g., without undo of any polarity inversion or lane reversal).The compliance pattern can be used to characterize operation of theanalog front end on some subset of lanes when Loopback is not functionalor undesired. For instance, entering Loopback can be preconditioned onsuccessful byte lock, TS lock, deskew, latency testing, and othertesting reliant on the proper functioning of several piece of digitallogic. With bit lock accomplished, Compliance can be entered and may beutilized for jitter or noise investigation, debug, exploring a link,among other conditioning. In other words, Compliance can serve as analternative exit for Polling in the event the results of Polling do notallow direct entry into Loopback. The Compliance state can drive asupersequence with a transmitter from the master. Receiver looks for awake on a monitor lane, debounces the wake, drops bad lanes, adapts, andbit locks, etc. The slave transmitter can drive the compliance patternuntil its receiver actions are complete. Then loop-back is re-timed andnon-deskewed. Slave receiver does similar monitor and debounce, etc.actions. Exit may be to a reset state, such as a timed reset, or to aLoopback Pattern state to start the test, among other examples. In thecase of an exit to Loopback Pattern state, the master may be sent (e.g.,by a software controller) to the Loopback Pattern state to try out morespecialized patterns.

Agent Loopback Marker State: Loopback marker is an agent state butunlike other agent states master and slave actions and exits may bedifferent. Loopback slave may undo any polarity inversion and/or lanereversal but may not descramble or rescramble looped back bits.Acknowledgment exchange may not apply to slave since it is looping back.Since slave may deskew before looping back on symbol boundary, mastermay not be forced to re-bytelock or re-deskew but the master may re-locktraining sequence to avoid locking to some alias. Means to do this mayinclude re-seeding of LFSR, comparing TS and/or EIEOS or somecombination of these. The end of the SDS marks the end of loopback setupand the start of pattern generation, checking and counting.

Agent Loopback Pattern State (or Block Link state): Entered from eithera Loopback Marker state or Compliance state. In Loopback Pattern state,instead of control patterns, a master transmitter can send additionalspecialized patterns to supplement those looped-back in a Compliance orLoopback Pattern state. The receiver can receive specialized patterns inLoopback Pattern and check for errors in the received pattern. Fortransmitter adaptation both agents can be masters. For a predeterminedperiod, the transmitter can sends a pattern and a remote receiver cancompare this pattern and determine a figure of merit or metric for thereceived pattern which is recorded in a storage element, such as aregister. The comparison method and metric may be design dependent(e.g., BER with jitter injection). At the end of the period, both agentscan exit to Reset for the backchannel to examine the metric and set upthe next iteration of transmitter adaptation.

Link width state: agent communicates with the final lane map to remotetransmitter. Receiver receives the information and decodes. Receiver mayrecord a configured lane map in a structure after checkpoint of aprevious lane map value in a second structure. Receiver may also respondwith an acknowledge (“ACK”). May initiate an in-band reset. As oneexample, first state to initiate in-band reset. In one embodiment, exitto a next state, such as flit configuration state, is performed inresponse to the ACK. Further, prior to entering low power state, a resetsignal may also be generated if the frequency of a wake detect signaloccurrence drops below a specified value (e.g. 1 every number of unitintervals (UIs), such as 4K UI). Receiver may hold current and previouslane maps. Transmitter may use different groups of lanes based ontraining sequences having different values. Lane map may not modify somestatus registers in some embodiments.

Flitlock configuration state: entered by a transmitter but the state isconsidered exited (i.e. secondary timeout moot) when both transmitterand receiver have exited to a blocking link state or other link state.Transmitter exit to a link state, in one embodiment, includes start of adata sequence (SDS) and training sequence (TS) boundary after receivinga planetary alignment signal. Here, receiver exit may be based onreceiving an SDS from a remote transmitter. This state may be a bridgefrom agent to link state. Receiver identifies SDS. Receiver may exit toblocking link state (BLS) (or a control window) if SDS received after adescrambler is initialized. If a timeout occurs, exit may be to resetstate. Transmitter drives lanes with a configuration signal. Transmitterexit may be to reset, BLS, or other states based on conditions ortimeouts.

Transmitting Link State: a link state. Flits are sent to a remote agent.May be entered from a blocking link state and return to a blocking linkstate on an event, such as a timeout. Transmitter transmits flits.Receiver receives flits. May also exit to a low power link state. Insome implementations, transmitting link state (TLS) can be referred toas the L0 state.

Blocking Link State: a link state. Transmitter and receiver areoperating in a unified manner. May be a timed state during which thelink layer flits are held off while the Physical layer information iscommunicated to the remote agent. May exit to a low power link state (orother link state based on the design). A blocking link state (BLS), inone embodiment, periodically occurs. The period is referred to as a BLSinterval and may be timed, as well as may differ between slow speed andoperational speed. Note that the link layer may be periodically blockedfrom sending flits so that a Physical layer control sequence of a lengthmay be sent, such as during a transmitting link state or a partial widthtransmitting link state. In some implementations, blocking link state(BLS) can be referred to as a L0 control, or L0c, state.

Partial Width Transmitting Link State: Link state. May save power byentering a partial width state. In one embodiment asymmetric partialwidth refers to each direction of a two direction link having differentwidths, which may be supported in some designs. An example of aninitiator, such as a transmitter, sending a partial width indication toenter partial width transmitting link state is shown in the example ofFIG. 9. Here, a partial width indication is sent while transmitting on alink with a first width to transition the link to transmit at a second,new width. A mismatch may result in a reset. Note that speeds may not bealtered but width may be. Therefore, flits are potentially sent atdifferent widths. May be similar to a transmitting link state logically;yet, since there is a smaller width, it may take longer to transmitflits. May exit to other link states, such as a low power link statebased on certain received and sent messages or an exit of the partialwidth transmitting link state or a link blocking state based on otherevents. In one embodiment, a transmitter port may turn idle lanes off ina staggered manner to provide better signal integrity (i.e. noisemitigation) as shown in the timing diagram. Here, non-retry-able flits,such as Null flits, may be utilized during periods where the link widthis changing. A corresponding receiver may drop these null flits and turnidle lanes off in a staggered manner, as well as record the current andprevious lane maps in one or more structures. Note status and associatedstatus register may remain unaltered. In some implementations, partialwidth transmitting link state can be referred to as a partial L0, orL0p, state.

Exit Partial Width Transmitting Link State: exit the partial widthstate. May or may not use a blocking link state in some implementations.The transmitter initiates exit, in one embodiment, by sending partialwidth exit patterns on the idle lanes to train and deskew them. As oneexample, an exit pattern start with EIEOS, which is detected anddebounced to signal that the lane is ready to start the entry to a fulltransmitting link state, and may end with SDS or Fast Training Sequence(FTS) on idle lanes. Any failure during the exit sequence (receiveractions, such as deskew not completed prior to timeout) stops flittransfers to the link layer and asserts a reset, which is handled byresetting the link on the next blocking link state occurrence. The SDSmay also initialize the scrambler/descrambler on the lanes toappropriate values.

Low Power Link State: is a lower power state. In one embodiment, it islower power than the partial width link state, since signaling in thisembodiment is stopped on all lanes and in both directions. Transmittersmay use a blocking link state for requesting a low power link state.Here, receiver may decode the request and respond with an ACK or a NAK;otherwise reset may be triggered. In some implementations, low powerlink state can be referred to as a L1 state.

In some implementations, state transitions can be facilitated to allowstates to be bypassed, for instance, when state actions of the states,such as certain calibrations and configurations, have already beencompleted. Previous state results and configurations of a link can bestored and reused in subsequent initializations and configurations of alink. Rather than repeating such configurations and state actions,corresponding states can be bypassed. Traditional systems implementingstate bypasses, however, often implement complex designs and expensivevalidation escapes. Rather than using a traditional bypass, in oneexample, HPI can utilize short timers in certain states, such as wherethe state actions do not need to be repeated. This can potentially allowfor more uniform and synchronized state machine transitions among otherpotential advantages.

In one example, a software-based controller (e.g., through an externalcontrol point for the Physical layer) can enable a short timer for oneor more particular states. For instance, for a state for which actionshave already been performed and stored, the state can be short-timed tofacilitate a quick exit from the state to a next state. If, however, theprevious state action fails or cannot be applied within the short timerduration, a state exit can be performed. Further, the controller candisable the short timer, for instance, when the state actions should beperformed anew. A long, or default, timer can be set for each respectivestate. If configuration actions at the state cannot be completed withinthe long timer, a state exit can occur. The long timer can be set to areasonable duration so as to allow completion of the state actions. Theshort timer, in contrast, may be considerably shorter making it, in somecases, impossible to perform the state actions without reference back topreviously-performed state actions, among other examples.

In some instances, during initialization (or re-initialization) of alink, as agents progress through a state machine toward an operationallink state, one or more failures or state exits can occur that cause thestate to reset (e.g., to a reset or other state). In effect, theinitialization of the link can loop through one or more states withoutcompleting the initialization and entering a link state. In one example,a count can be maintained for the number of unproductive loops in statetransitions within the initialization of a link. For instance, each timean initialization returns to a reset state without reaching a link statea counter can be incremented. The counter can be reset for the link oncethe link successfully enters a link state. Such counters can bemaintained by agents on both sides of the link. Further, a threshold canbe set, for instance, by a software-based controller utilizing one ormore external control points. When the count of unproductive loops meets(or exceeds) the defined threshold initialization of the link can besuspended (e.g., set and held at or before the reset state). In someimplementations, in order to recommence initialization and release theinitialization from the suspended state, a software-based controller cantrigger a restart or re-initialization of the link. In some instances,the software-based tools can analyze the nature of the suspendedinitialize and perform diagnostics, set register values, and performother operations so as to guard against further looping of theinitialization. Indeed, in some implementations, a controller can set ahigher counter threshold or even override the counter, among otherexamples, in connection with restarting a suspended link initialization.

In some implementations of HPI, supersequences can be defined, eachsupersequence corresponding to a respective state or entry/exit to/fromthe respective state. A supersequence can include a repeating sequenceof data sets and symbols. The sequences can repeat, in some instances,until completion of a state or state transition, or communication of acorresponding event, among other examples. In some instances, therepeating sequence of a supersequence can repeat according to a definedfrequency, such as a defined number of unit intervals (UIs). A unitinterval (UI) can correspond to the interval of time for transmitting asingle bit on a lane of a link or system. In some implementations, therepeating sequence can begin with an electrically ordered set (EOS).Accordingly, an instance of the EOS can be expected to repeat inaccordance with the predefined frequency. Such ordered sets can beimplemented as defined 16 Byte codes that may be represented inhexadecimal format, among other examples. In one example, the EOS of asupersequence can be an EIEIOS. In one example, an EIEOS can resemble alow frequency clock signal (e.g., a predefined number of repeating FF00or FFF000 hexadecimal symbols, etc.). A predefined set of data canfollow the EOS, such as a predefined number of training sequences orother data. Such supersequences can be utilized in state transitionsincluding link state transitions as well as initialization, among otherexamples.

In some implementations of an interconnect, such as in QPI, terminationsof a serial data link can be brought on and off, such as when a link isreset or initialized. This approach can introduce complexity and timeinto the initialization of a link. In some implementations of HPI,terminations of the link can be maintained including during a reset orre-initialization of the link. Further, HPI can permit hot-plugging ofdevices. When another device is introduced, either through hot-pluggingor otherwise, the voltage characteristics of the lane on which the newremote agent is added will change. The local agent can sense thesechanges in the lane voltage to detect the presence of the remote agentand prompt initialization of the link. State machine states and timerscan be defined in the state machine to coordinate the detection,configuration, and initialization of a link without terminations.

In one implementation, HPI can support re-initialization on an in-bandreset without changing the termination values through the screening of alane by the receiving agent for incoming signaling. The signaling can beused to identify good lanes. As an example, the lane can be screened forany one of a set of pre-defined signals that are to be sent by atransmitter device to facilitate discovery and configuration of thelink. In one example, a supersequence can be defined corresponding toone or more initialization or re-initialization tasks. The pre-definedsequence can include an electric idle exit ordered set (EIEOS) followedby additional sequence data. In some instances, as each device on eitherside of a lane becomes active, the devices can begin sending asupersequence corresponding to a particular initialization state, etc.In one embodiment, two types of pin resets can be supported; power-on(or “cold”) reset and warm reset. A reset initiated by software ororiginating (in the Physical or another layer) on one agent may becommunicated in-band to the other agent. However, due to usage of anembedded clock, an in-band reset may be handled by communication toanother agent using an ordered set, such as a specific electricalordered set or EIOS.

The ordered set can be sent during initialization and a PHY controlsequence (or “blocking link state”) can be sent after initialization.The block link state can block the link layer from sending flits. Asanother example, link layer traffic may be blocked to send a few NULLflits which may be discarded at the receiver.

As introduced above, initialization, in one embodiment, can be doneinitially at slow speed followed by initialization at fast speed.Initialization at slow speed uses the default values for the registersand timers. Software then uses the slow speed link to setup theregisters, timers and electrical parameters and clears the calibrationsemaphores to pave the way for fast speed initialization. As oneexample, initialization can consist of such states or tasks as Reset,Detect, Polling, Compliance, and Configuration, among potentiallyothers.

In one example, a link layer blocking control sequence (i.e. a blockinglink state (BLS) or LOc state) can include a timed state during whichthe link layer flits are held off while the PHY information iscommunicated to the remote agent. Here, the transmitter and receiver maystart a block control sequence timer. And upon expiration of the timers,the transmitter and receiver can exit the blocking state and may takeother actions, such as exit to reset, exit to a different link state (orother state), including states that allow for the sending of flitsacross the link.

In one embodiment, link training can be provided and include the sendingof one or more of scrambled training sequences, ordered sets, andcontrol sequences, such as in connection with a defined supersequence. Atraining sequence symbol may include one or more of a header, reservedportions, a target latency, a pair number, a physical lane map codereference lanes or a group of lanes, and an initialization state. In oneembodiment, the header can be sent with a ACK or NAK, among otherexamples. As an example, training sequences may be sent as part ofsupersequences and may be scrambled.

In one embodiment, ordered sets and control sequences are not scrambledor staggered and are transmitted identically, simultaneously andcompletely on all lanes. A valid reception of an ordered set may includechecking of at least a portion of the ordered set (or entire ordered setfor partial ordered sets). Ordered sets may include an electricallyordered set (EOS), such as an Electrical Idle Ordered Set (EIOS) or anEIEOS. A supersequence may include a start of a data sequence (SDS) or aFast Training Sequence (FTS). Such sets and control supersequences canbe predefined and may have any pattern or hexadecimal representation, aswell as any length. For example, ordered sets and supersequences may bea length of 8 bytes, 16, bytes, or 32 bytes, etc. FTS, as an example,can additionally be utilized for fast bit lock during exit of a partialwidth transmitting link state. Note that the FTS definition may be perlane and may utilize a rotated version of the FTS.

Supersequences, in one embodiment, can include the insertion of an EOS,such as an EIEOS, in a training sequence stream. When signaling starts,lanes, in one implementation, power-on in a staggered manner. This mayresult, however, in initial supersequences being seen truncated at thereceiver on some lanes. Supersequences can be repeated however overshort intervals (e.g., approximately one-thousand unit intervals (or ˜1KUI)). The training supersequences may additionally be used for one ormore of deskew, configuration and for communicating initializationtarget, lane map, etc. The EIEOS can be used for one or more oftransitioning a lane from inactive to active state, screening for goodlanes, identifying symbol and TS boundaries, among other examples.

In one embodiment, the clock can be embedded in the data so there are noseparate clock lanes. The flits sent over the lanes can be scrambled tofacilitate clock recovery. The receiver clock recovery unit, as oneexample, can deliver sampling clocks to a receiver (i.e. the receiverrecovers clock from the data and uses it to sample the incoming data).Receivers in some implementations continuously adapt to an incoming bitstream. By embedding the clock, pinout can be potentially reduced.However, embedding the clock in the in-band data can alter the manner inwhich in-band reset is approached. In one embodiment, a blocking linkstate (BLS) can be utilized after initialization. Also, electricalordered set supersequences may be utilized during initialization tofacilitate the reset (e.g., as described above), among otherconsiderations. The embedded clock can be common between the devices ona link and the common operational clock can be set during calibrationand configuration of the link. For instance, HPI links can reference acommon clock with drift buffers. Such implementation can realize lowerlatency than elastic buffers used in non-common reference clocks, amongother potential advantages. Further, the reference clock distributionsegments may be matched to within specified limits.

In some implementations, HPI may support flits with a width that is, insome cases, not a multiple of the nominal lane width (e.g. using a flitwidth of 192 bits and 20 lanes as a purely illustrative example).Indeed, in implementations permitting partial width transmitting states,the number of lanes over which flits are transmitted can fluctuate, evenduring the life of the link. For example, in some instances, the flitwidth may be a multiple of the number of active lanes at one instant butnot be a multiple of the number of active lanes at another instant(e.g., as the link changes state and lane width). In instances where thenumber of lanes is not a multiple of a current lane width (e.g., theexample of a flit width of 192 bits on 20 lanes), in some embodiments,consecutive flits can be configured to be transmitted to overlap onlanes to thereby preserve bandwidth (e.g., transmitting five consecutive192 bit flits overlapped on the 20 lanes).

FIG. 8 illustrates a representation of transmission of consecutive flitsoverlapped on a number of lanes. For instance, FIG. 8 shows arepresentation of five overlapping 192-bit flits sent over a 20 lanelink (the lanes represented by columns 0-19). Each cell of FIG. 8represents a respective “nibble” or grouping of four bits (e.g., bits 4n+3:4 n) included in a flit sent over a 4 UI span. For instance, a 192bit flit can be divided into 48 four-bit nibbles. In one example, nibble0 includes bits 0-3, nibble 1 includes bits 4-7, etc. The bits in thenibbles can be sent so as to overlap, or be interleaved (e.g.,“swizzled”), such that higher-priority fields of the flit are presentedearlier, error detection properties (e.g., CRC) are retained, amongother considerations. Indeed, a swizzling scheme can also provide thatsome nibbles (and their respective bits) are sent out of order (e.g., asin the examples of FIGS. 8 and 9). In some implementations, a swizzlingscheme can be dependent on the architecture of the link layer and formatof the flit used in the link layer.

The bits (or nibbles) of a flit with a length that is not a multiple ofthe active lanes can be swizzled, such as according to the example ofFIG. 8. For instance, during the first 4 UI, nibbles 1, 3, 5, 7, 9, 12,14, 17, 19, 22, 24, 27, 29, 32, 34, 37, 39, 42, 44 and 47 can be sent.Nibbles 0, 2, 4, 6, 8, 11, 13, 16, 18, 21, 23, 26, 28, 31, 33, 36, 38,41, 43, and 46 can be sent during the next 4 UI. In UIs 8-11, only eightnibbles remain of the first flit. These final nibbles (i.e., 10, 15, 20,25, 30, 40, 45) of the first flit can be sent concurrently with thefirst nibbles (i.e., nibbles 2, 4, 7, 9, 12, 16, 20, 25, 30, 35, 40, 45)of the second flit, such that the first and second flits overlap or areswizzled. Using such a technique, in the present example, five completeflits can be sent in 48 UI, with each flit sent over a fractional 9.6 UIperiod.

In some instances, swizzling can result in periodic “clean” flitboundaries. For instance, in the example of FIG. 8, the starting 5-flitboundary (the top line of the first flit) may also be referred to as aclean flit boundary since all lanes are transmitting starting nibblefrom same flit. Agent link layer logic can be configured to identifyswizzling of lanes and can reconstruct the flit from the swizzled bits.Additionally, physical layer logic can include functionality foridentifying when and how to swizzle a stream of flit data based on thenumber of lanes being used at the moment. Indeed, in a transition fromone link width state to another, agents can configure themselves toidentify how swizzling of the data stream will be employed.

As noted above, links can transition between lane widths, in someinstances operating at an original, or full, width and latertransitioning to (and from) a partial width utilizing fewer lanes. Insome instances, the defined width of a flit may be divisible by thenumber of lanes. For instance, the example of FIG. 9 illustrates such anexample, where the 192-bit flit of the previous examples is transmittedover an 8-lane link. As represented in FIG. 9, 4-bit nibbles of a192-bit flit can be evenly distributed and transmitted over 8 lanes(i.e., as 192 is a multiple of 8). Indeed, a single flit may be sentover 24 UI when operating at an 8-lane partial width. Further, each flitboundary can be clean in the example of FIG. 9. While clean flitboundaries can simplify the state transitions, determinism, and otherfeatures, allowing for swizzling and occasional jagged flit boundariescan allow for the minimization of wasted bandwidth on a link.

Additionally, while the example of FIG. 9, shows lanes 0-7 as the lanesthat remained active in a partial width state, any set of 8 lanes canpotentially be used. Note also that the examples above are for purposesof illustration only. The flits can potentially be defined to have anywidth. Links can also have potentially any link width. Further, theswizzling scheme of a system can be flexibly constructed according tothe formats and fields of the flit, the preferred lane widths in asystem, among other considerations and examples.

The operation of the HPI PHY logical layer can be independent of theunderlying transmission media provided the latency does not result inlatency fixing errors or timeouts at the link layer, among otherconsiderations.

Link Layer

The Link layer can abstract the Physical layer from the Protocol layer,handle flow control between two protocol agents, and provide virtualchannel services to the Protocol layer (Message Classes) and Routinglayer (Virtual Networks). In some implementations, the Link layer candeal with a fixed quantum of information, termed a flit. In one example,the flit can be defined to be 192 bits in length. However, any range ofbits, such as 81-256 (or more) may be utilized in different variations.A large flit size, such as 192 bits, may include format, cyclicredundancy check (CRC), error correction code (ECC), and other features.For instance, a larger flit length can also permit the CRC field to beexpanded (e.g., to 16 bits) to handle the larger flit payload. Thenumber of phits or unit intervals (UI) (e.g., the time used to transfera single bit or phit, etc.) to transfer a single flit can vary with linkwidth. For instance, a 20 lane or bit link width can transfer a single192 bit flit in 9.6 UI, while an 8 lane link width transfers the sameflit in 24 UI, among other potential examples. The link layer creditingand protocol packetizing can also be based on a flit.

FIG. 10 illustrates a representation 1000 of a generalized flit for an 8lane link width. Each column of the representation 1000 can symbolize alink lane and each row a respective UI. In some implementations, asingle flit can be subdivided into two or more slots. Distinct messagesor link layer headers can be included in each slot, allowing multipledistinct, and in some cases, independent messages corresponding topotentially different transactions to be sent in a single flit. Further,the multiple messages included in slots of a single flit may also bedestined to different destination nodes, among other examples. Forinstance, the example of FIG. 10 illustrates a flit format with threeslots. The shaded portions can represent the portion of the flitincluded in a respective slot.

In the example of FIG. 10, three slots, Slots 0, 1, and 2, are provided.Slot 0 can be provided 72 bits of flit space, of which 22 bits arededicated to message header fields and 50 bits to message payload space.Slot 1 can be provided with 70 bits of flit space, of which 20 bits arededicated to message header fields and 50 bits to message payload space.The difference in message header field space between can be optimized toprovide that certain message types will be designated for inclusion inSlot 0 (e.g., where more message header encoding is utilized). A thirdslot, Slot 2, can be provided that occupies substantially less spacethan Slots 0 and 1, in this case utilizing 18 bits of flit space. Slot 2can be optimized to handle those messages, such as acknowledges, creditreturns, and the like that do no utilize larger message payloads.Additionally, a floating payload field can be provided that allows anadditional 11 bits to be alternatively applied to supplement the payloadfield of either Slot 0 or Slot 1.

Continuing with the specific example of FIG. 10, other fields can beglobal to a flit (i.e., apply across the flit and not to a particularslot). For instance, a header bit can be provided together with a 4-bitflit control field that can be used to designate such information as avirtual network of the flit, identify how the flit is to be encoded,among other examples. Additionally, error control functionality can beprovided, such as through a 16-bit cyclic CRC field, among otherpotential examples.

A flit format can be defined so as to optimize throughput of messages onthe Link layer. Some traditional protocols have utilized unslotted,smaller flits. For instance, in QPI an 80-bit flit was utilized. Whilethe flit throughput of a larger (e.g., 192-bit flit) may be lower,message or packet throughput can be increased by optimizing use of theflit data. For instance, in some implementations of QPI, the entire80-bit flit space was utilized regardless of the message size or type.By subdividing a larger flit into slots of predetermined lengths andfields, the 192 flit length can be optimized realizing higher efficiencyeven in instances when one or more of the available slots are sometimesunused. Indeed, Link layer traffic can be assumed to include manydifferent types of messages and traffic, including messages and packetswith varying header lengths and fields. The respective lengths andorganization of slots defined in a flit can be defined so as tocorrespond with the statistical or expected frequency of variousmessages and the needs of these messages. For instance, two larger slotscan be defined for every small slot, to accommodate an expectedstatistical frequency of messaging using these larger message types andheader lengths, among other example. Further, flexibility can also beprovided to further accommodate the varied traffic, such as through afloating payload field, as in the example of FIG. 10. In some instances,a flit format can be fixed, including the bits dedicated to particularslots in the flit.

In the example of FIG. 10, a “Hdr” field can be provided for the flitgenerally and represent a header indication for the flit. In someinstances, the Hdr field can indicate whether the flit is a header flitor a data flit. In data flits, the flit can still remain slotted, butomit or replace the use of certain fields with payload data. In somecases, data fields may include an opcode and payload data. In the caseof header flits, a variety of header fields can be provided. In theexample of FIG. 10, “Oc” fields can be provided for each slot, the Ocfield representing an opcode. Similarly, one or more slots can have acorresponding “msg” field representing a message type of thecorresponding packet to be included in the slot, provided the slot isdesigned to handle such packet types, etc. “DNID” fields can represent aDestination Node ID, a “TID” field can represent a transaction ortracker ID, a “RHTID” field can represent either a requestor node ID ora home tracker ID, among other potential fields. Further, one or moreslots can be provided with payload fields. Additionally, a CRC field canbe included within a flit to provide a CRC value for the flit, amongother examples.

In some implementations, link width can vary during the life of thelink. For instance, the Physical layer can transition between link widthstates, such as to and from a full or original lane width and adifferent or partial lane width. For example, in some implementations, alink can be initialized to transfer data over 20 lanes. Later, the linkcan transition to a partial width transmitting state where only 8 lanesare actively used, among many other potential examples. Such lane widthtransitions can be utilized, for instance, in connection with powermanagement tasks governed by one or more power control units (PCU) amongother examples.

As noted above, link width can influence flit throughput rate. FIG. 11is a representation of an example 192-bit flit sent over an 8 lane link,resulting in throughput of the flit at 24 UI. Further, as shown in theexample of FIG. 11, bits of the flit can be sent out of order in someinstances, for example, to send more time-sensitive fields earlier inthe transfer (e.g., flit type fields (e.g., data or header flit),opcodes, etc.), preserve or facilitate particular error detection orother functionality embodied in the flit, among other examples. Forinstance, in the example of FIG. 11, bits 191, 167, 143, 119, 95, 71,47, and 23 are sent in parallel on lanes L7 through L0 during a first UI(i.e., UI0) of transfer, while bits 168, 144, 120, 96, 72, 48, 24, and 0are sent during the 24^(th) (or final) UI of the flit transfer (i.e.,UI23). It should be appreciated that other ordering schemes, flitlengths, lane widths, etc. can be utilized in other implementations andexamples.

In some instances, the length of the flit can be a multiple of thenumber of active lanes. In such instances, the flit can be transmittedevenly on all active lanes and transfer of the flit can endsubstantially simultaneously at a clean (i.e., non-overlapping)boundary. For example, as shown in the representation of FIG. 8, bits ofa flit can be considered to be transmitted in consecutive groupings of 4bits, or “nibbles.” In this example, a 192 bit flit is to be transferredover an 8 lane link. As 192 is a multiple of 8, the entire flit can becleanly transferred over the 8 lane link in 24 UI. In other instances,the flit width may not be a multiple of the number of active lanes. Forinstance, FIG. 9 shows another representation of an example 192 bittransferred over 20 lanes. As 192 is not evenly divisible by 20,transfer of the full flit would require a non-integer number ofintervals (e.g., 9.6 UI). In such cases, rather than wasting “extra”lanes not utilized during the 10th UI of transfer, a second overlappingflit can be transferred with the final bits of a preceding flit. Suchoverlapping, or swizzling, of the flits can result in jagged flitboundaries and flit bits sent out of order in some implementations. Thepattern utilized for the transfer can be configured to allow moretime-sensitive fields of the flit to be transferred earlier in the flit,preservation of error detection and correction, among otherconsiderations. Logic can be provided in one or both of the Physical andLink layers to transfer flit bits according to such patterns anddynamically change between patterns based on the current link width.Further logic can be provided to re-order and re-construct flits fromsuch swizzled or ordered bit streams, among other examples.

In some implementations, flits can be characterized as header flits(e.g., bearing packet header data) or data flits (e.g., bearing packetpayload data). Returning to FIG. 10, a flit format can be defined thatincludes three (3) distinct slots (e.g., 0, 1, and 2), allowing up tothree headers to be transferred in a single flit (e.g., one header ineach slot). In the example of FIG. 10, three slots, Slots 0, 1, and 2,are provided. Slot 0 can be provided 72 bits of flit space, of which 22bits are dedicated to message header fields and 50 bits to messagepayload space. Slot 1 can be provided with 70 bits of flit space, ofwhich 20 bits are dedicated to message header fields and 50 bits tomessage payload space. The difference in message header field spacebetween can be optimized to provide that certain message types will bedesignated for inclusion in Slot 0 (e.g., where more message headerencoding is utilized). A third slot, Slot 2, can be provided thatoccupies substantially less space than Slots 0 and 1, in this caseutilizing 18 bits of flit space. Slot 2 can be optimized to handle thosemessages, such as acknowledges, credit returns, and the like that do noutilize larger message payloads. Additionally, a floating payload fieldcan be provided that allows an additional 11 bits to be alternativelyapplied to supplement the payload field of either Slot 0 or Slot 1.

In some implementations, by allowing a field to float between the twoslots, extra bits can be provided as needed for certain messages whilestill staying within a predefined flit length (e.g., 192 bits) andmaximizing the utilization of the bandwidth. Turning to the examples ofFIG. 19, two instances 1905, 1910 of an example 192-bit flit are shownon an 8 lane data link. In one instance, a flit (e.g., 1905) can includethree slots, Slots 0, 1, and 2. Each of Slots 0 and 1 can include 50-bitpayload fields. The floating field can be provided to alternativelyextend the payload field of the either Slot 0 or Slot 1 by the fieldlength (e.g., 11 bits) of the floating field. The use of a floatingfield can further extend the efficiency gains provided through adefined, multi-slot flit format. The sizing of the slots within theflit, and the types of messages that can be placed in each slot, canpotentially provide increased bandwidth even with a reduced flit rate.

In the particular example of FIG. 10, the messages that can use Slots 1and 2 can be optimized, reducing the number of bits to be set aside toencode these slots' opcodes. When a header having more bits that Slot 0can provide enters the Link layer, slotting algorithms can be providedto allow it to take over Slot 1 payload bits for additional space.Special control (e.g. LLCTRL) flits may also be provided that consumeall three slots worth of bits for their needs. Slotting algorithms mayalso exist to allow individual slots to be utilized while other slotscarry no information, for cases where the link is partially busy.

In one embodiment, the Link Layer can additionally define specialcontrol flits that may be used, for instance, for debug messages andother uses. In one example, LLCTRL-DEBUG flits may be sent by the LinkLayer when an enable debug control field is set. When this bit is notset, LLCTRL-DEBUG flits may not be transmitted on the link. DebugPackets may be important to expose internal states of devices connectedby HPI that are otherwise inaccessible. The contents of debug packetscan also be implementation specific. Contents could include things likebranch info (source and target IPs), time-stamps, indication of aninternal event trigger, and so on. The exposed data can be, forinstance, by monitoring devices such as logic analyzers forpost-processing and failure analysis. An example flit encoding of aDebug message type is illustrated in FIG. 12.

In some implementations, principles and protocols of a general purposeI/O interconnect (GPIO) architecture, such as HPI, can be utilized toimplement a buffered memory interface and corresponding protocol. Forinstance, the physical and link layer definitions outlined above canalso be implemented in a buffered memory protocol. Indeed, logic used tosupport the physical and link layers of the GPIO protocol can be reusedat interfaces supporting the buffered memory protocol. The bufferedmemory protocol can also share message classes, such as a request,response, and writeback message class, among other examples. Whileopcode values within the buffered memory protocol message can beinterpreted differently than in HPI (or another GPIO protocol), the samegeneral packet formats can be utilized in both the buffered memoryprotocol and the GPIO interconnect upon which it is built. Indeed,additional packet and flit fields can be unique to both the bufferedmemory protocol and GPIO interconnect protocol.

In some implementations, a buffered memory protocol utilizing thephysical and link layers of HPI can be a transactional interface andprotocol. This can permit data to be returned out of order to a seriesof received requests (e.g., read requests). The buffered memoryinterface can be used to interconnect a buffer chip to a CPU in someimplementations. In one implementation, While some protocols send DRAMcommands such as Activate, Read, Write, and Refresh across a bufferedmemory bus, in some implementations of the buffered memory protocoldiscussed herein, a read or write command can simply be sent with anaddress. The buffer chip can then decode the specific rank and bank, andbreaks the request down to DRAM commands. Speculative and demand readscan also be supported.

Turning to FIG. 13, a simplified block diagram 1300 is shownillustrating an example topology of a computing system including CPUdevices 1305, 1310 interconnected by an HPI-compliant link. Each CPU1305, 1310 can be likewise connected to one or more respective bufferdevices 1315 a-1 using corresponding buffered memory protocol links(“MemLink”). As noted above, in some implementations, the bufferedmemory protocol interconnect can be based on the GPIO protocol, in thatthe physical and link layers of the buffered memory protocols are basedon the same physical and link layer definitions of the GPIO protocol(e.g., HPI). Although not illustrated in FIG. 13, the CPUs 1305, 1310can be further connected to one or more downstream devices using theGPIO protocol.

As further shown in the example of FIG. 13, buffer devices 1315 a-1 canbe connected to memory devices, such as dual in-line memory module(DIMM) devices. The memory corresponding to each buffer device can beconsidered local to the CPU (e.g., 1305, 1301) to which the bufferdevice is connected. However, other devices (including the other CPU)can access the memory by other sockets using GPIO protocol-compliantlinks. In some implementations, a port running the buffered memoryprotocol may only support the commands for communicating with the memoryand only support the buffered memory protocol (i.e., not the GPIOprotocol and the buffered memory protocol). Additionally, in someimplementations, the GPIO interconnect protocol may support routing andindicate such information (e.g., in its packets) such as the requestingand destination node identifiers. The buffered memory protocol, on theother hand, may be a point-to-point interface that does not utilizerouting. Consequently, some fields used in the GPIO protocol may bedispensed with in packets sent using the buffered memory interfaces.Instead, fields can be designated for use in carrying address decodeinformation host to buffer, among other examples.

In further implementations, buffer devices 1315 a-1 can support a twolevel memory topology with some amount of fast memory (e.g., DRAM)serving as a cache for a larger, slower memory (e.g., non-volatilememory). In one such implementation, one or more of the buffer devices1315 a-1 can use DDR as near, fast memory and transactional DDR DIMMs asthe larger “far” memory, among other examples. Transactional DIMMs canutilize protocols (e.g., DDR-Transactional (DDR-T)) to communicate to avolatile memory single in-line memory module (SIMM) using transactionalcommands.

Turning to FIG. 14, a simplified block diagram 1400 is presented showinga more detailed representation of a buffer chip 1315 connected to nearand far memory modules (e.g., 1405, 1410). As noted in the exampleabove, the buffer chip 1315 can be communicatively coupled to a CPUdevice or other processing device 1305 through a buffered memoryprotocol interconnect link 1415 utilizing a defined buffered memoryprotocol. The buffer chip 1315 can include logic to perform additionalfunctions. For instance, an implementation of the buffer chip 1315 caninclude a memory controller 1420, a scheduler 1425, flow control logic1430, and error detection logic 1435. Logic can be implemented usinghardware circuitry, firmware, and/or software.

In some implementations, system memory controlled by the memorycontroller can be implemented in a two layer memory (2LM) system memoryscheme and include near 1405 and far memory 1410. For instance, a memorytopology can use a relatively small amount of fast memory (e.g., DRAM orembedded DRAM) as a cache for a much larger, slower memory (e.g.,implemented as non-volatile memory, such as NAND flash memory) that isdenser and can consume less power than DRAM. Such an implementation ofsystem memory can realize comparatively more storage and less powerconsumption than traditional topologies, for instance, due to the use ofdenser, less power hungry technology for far memory 1410. The penalty ofusing slower phase change technology is compensated for at leastsomewhat by using a layer of DRAM in near memory 1405 as a caching layerfor far memory 1415. Further, in some implementations, if morefrequently used information is kept in near memory 1405 DRAM the accesstimes of reaching such information can also be improved.

In one example, the near memory 1405 can act a direct mapped cache tothe far memory 1410 in 2LM. In one implementation, cache tags can bestored in the error correction code (ECC) bits of the near memory DRAMdata. Further, in one embodiment, near memory 1405 and far memory 1410can both appear on Double Data Rate 4 (DDR4) busses connected to thebuffer chip 1315. Near memory 1405 can run the standard DDR4 protocoland far memory 1410, implemented as one or more DIMMs, can run DDR-basedprotocol. Near memory 1405 can be installed on all DDR4 busses and farmemory 1410 can be installed only on one or more DDR4 busses. MultipleDRAM busses can be controlled by one memory controller. Near and farmemory may be on the same or different DRAM busses. In oneimplementation, all of the DRAM busses on a memory buffer chip 1315 canbe behind the same memory controller. This can be implemented asgenerally two or more busses. Further, near memory 1405 caches the farmemory 1410 behind the same buffer chip 1315.

As noted, in some implementations, near memory 1405 can be a directmapped cache. In the case of a direct mapped cache, a set of systemmemory addresses (in far memory) map to a single entry in near memory(the location being determinable from the system memory addressesthemselves). Thus, in a sense, more than one system memory address willcompete for use of a same near memory entry.

In operation, for any read or write request, a memory controller 1420can perform a read into near memory (e.g., 1405) to see if the cacheline to which the request pertains is cached there. If so (i.e., a nearmemory cache “hit”), the memory controller 1420 performs the requestedoperation and keeps the cache line in near memory 1405. In the case of aread, a copy of the requested cache line is sent to the requestor andthe cache line is not written over in near memory (i.e., it is keptthere). In the case of a write, the new data for the cache line that wasreceived in the request is written over the old cache line in nearmemory.

If the requested cache line is not found in near memory (i.e., a nearmemory cache “miss”), in the case of a read, the memory controller readsthe cache line from far memory (e.g., 1410), sends a copy of the cacheline to the requestor and writes the cache line into near memory. In thecase of a write, when the requested cache line is not found in nearmemory, the memory controller simply writes the cache line that wasreceived with the request into far memory. No copy is placed in nearmemory. Thus, if there is no “hit” in near memory, a read will evict thecache line in near memory but a write will not evict the cache line innear memory.

When it is possible that a copy of a writable cache line (e.g., a cacheline in a modified (“M”) or exclusive (“E”) state) within one of thecaches of the system might not exist in the near memory, the system issaid to be operating in a “non-inclusive” mode. Here, “non-inclusive”mode demands a read into near memory for any write operation to systemmemory, even for cache lines in the M or E state in a processor cache,because it is not known whether or not a copy of the cache line residesin near memory. This lack of knowledge can result in systeminefficiencies.

Another mode of operation, referred to as “inclusive” can guarantee thata writable cache line in any processor caching level will be kept innear memory. This means, for instance, if an M or E cache line is cachedwithin a processor, a copy or older version of the cache line for thesame system memory address is also kept in near memory. Thus if an M/Ecache line in a processor cache is written back, the written back cacheline is directly written into near memory by the memory controller. Inother words, a read of near memory need not be performed beforehand.Cache lines can and will be evicted from near memory, however. Forexample, if a read request is received by a memory controller for acache line having a different system memory address than the address ofa cache line in near memory in an M/E state in a system level cache, butthat still maps to the same location in near memory, a near memory cachemiss will result. The cache line in near memory will be evicted fromnear memory and written back to far memory if the line is “dirty” innear memory. The requested cache line can then be read from far memoryand forwarded to the requestor. The requested cache line will also bewritten into near memory as discussed above (a read on a near memorymiss will evict the near memory cache line). Accordingly, a cache linein near memory having a related cache line in the M or E state in asystem level cache can be evicted from near memory in violation of an“inclusive” write guarantee for the system. Accordingly, a hostcontroller (e.g., 1440) and/or a memory controller (e.g., 1420) can beprovided with logic to address this issue. For instance, a coherenceagent in the host controller can track near memory conflicts and ensurethat any line contained in the E or M state of any CPU cache is residentin the 2LM cache. Whether a line is operating in connection with an“inclusive” or “non-inclusive” mode can also be tracked in the CPUcache. In some implementations, principles and solutions discussed inU.S. patent application Ser. No. 14/142,045, entitled “Method andApparatus for Eliminating Tag Reads for Writes in Two-Level Memory,”filed Dec. 27, 2013 and incorporated herein by reference, can beutilized, among other examples.

In some implementations, the memory controller 1420 can include logic toconvert requests into the memory specific protocol (e.g., DDR4) of itsmemory devices. The memory controller 1420 can abstract the details ofthese memory-specific protocols from the host controller logic 1440 ofthe CPU 1305 (or another host) which communicates with the buffer chip1315 using link 1415. Scheduler 1425 can include logic to reorder andarbitrate responses to requests in order to achieve higher performance.Buffer chip 1315 can additionally offer features such as flow controland error detection. In some implementations, flow control logic 1430can be embodied in scheduler 1425 or otherwise interoperate withscheduler 1425 to ensure more efficiency and higher performance of thebuffer chip 1315. Error detection logic 1435 can include logic tosupport an error correction code (ECC) detection scheme as well as logicfor initiating correction or handling of detected errors. Directorystate management logic 1436 can maintain the directory state of eachline in memory. In response to some transactions, the directory state ofa corresponding line can change. The directory state management logic1436 can update records identifying the current directory state of eachline in memory. In some cases, the directory state management logic 1436can receive data from a home agent (e.g., at host 1305) to indicate achange to the directory state of a line of memory. In other instances,directory state management logic 1436 can change include logic toautomatically update the directory state (i.e., without the hostindicating the change) based on the nature of a corresponding request.This can save bandwidth, among other example advantages, as bandwidthused to indicate at least a portion of the directory state changes canbe preserved, with the directory state management logic 1436 allowing aportion of these directory state changes to be identified directly atthe memory controller (e.g., 1420 at 1315).

In some implementations, CPU 1305 can include host controller logic 1440that can translate CPU (or host) addresses to memory addresses,including permuting across channels and sockets, among otherfunctionality. The host 1305 can include multiple processing cores 1445a-d that can allow multiple processes to execute in parallel on the host1305. Additionally, the host can make use of one or more caches to cachepreviously accessed lines from memory (e.g., 1405, 1410) such that thehost 1305 can re-accesses these lines without re-requesting the datafrom the buffer chip 1315. This can assist with addressing higherlatency introduced through the buffer chip 1315 in a buffered memoryarchitecture. Caches can include local and intermediate caches, as wellas last level cache (LLC) 1450 a-d. Cache, including LLC, can be sharedbetween multiple cores 1445 a-d. Indeed, a variety of cachingarchitectures can be utilized with hosts employing one or multipleprocessing cores.

As noted above, the link layer of a buffered memory protocol can bebased on that of a GPIO protocol. For instance, the 192-bit, 3-slot flitof an HPI protocol (e.g., described and shown in FIG. 10) can beutilized in a buffered memory protocol. A packet in the buffered memoryprotocol can embody an entire command. A packet can be implemented asone or more flits. FIGS. 15A-15C can represent implementations of a flitin a buffered memory protocol. In some implementations, read data can besent back from the buffer device to a host via a combination of headeredand headerless memory data (MemData) packets. An entire cache line canbe transferred in three headerless flits (rather than five, as withheadered flits). A headerless packet can thereby provide for higherthroughput on what may be the most constrained portion of the interface.In order to fit into three flits, the headerless packet removes a numberof fields and makes assumptions as to their values. The assumptions mademay be true for most, but all read returns, If any assumptions are nottrue, the headered packet is to be used, which contains all fields. Insome implementations, a headered packet can begin a sequence of readreturns to permit early identification of a tracker identifier (e.g.,HTID, RTID, etc.) for use in mapping a read return to a read request ina transactional buffered memory protocol.

In some implementations, headerless packets can contain the informationoutlined in Table 1:

TABLE 1 Headerless Packet Fields Field Qty Description Data 512 64 bytecache line data Directory 2 Two directory bits, which are stored withthe data, generally as part of the ECC bits HTID 11 Request transactionID. Due to early HTID, the packet contains the HTID for the next readdata packet CRC 48 16 bits per of CRC per flit HIB 3 Header indicationbit. One per flit. This bit is always 0 for each flit of headerlesspackets.Further, FIGS. 15A-15C illustrate a sequence of three flits (e.g., flit0 (at FIG. 15A), flit 1 (at FIG. 15B), and flit 2 (at FIG. 15C)) thatcan be used to transmit an entire cache line in a read return. Table 2provides a summary of an example of the three flits utilized in aheaderless packet:

TABLE 2 Headerless Packet Flit Contents Data flit 0 Data from the first32 bytes of the cache line. Data flit 1 Remainder of the first 32 bytes,plus the first part of the second 32 bytes. Data flit 2 Remainder of thesecond 32 bytes

An HIB bit can be set to indicate whether the packet is headered orheaderless. HIB may be set to indicate a headerless packet only for dataflits following a header flit. This allows certain other packets to beinterleaved in the middle of a packet. When the interleaved packet iscompleted, the data portions of the original headered packet can be sentwith HIB=0 in the flits. This technique can also be used to delay thedata packets by inserting null flits. In some implementations, the startof a headerless packet is signaled by the HIB bit being a 0 whennormally a header would be required. All other packets, including nullflits, may contain a header with the HIB bit set.

As introduced above, headered packets may be a total of five flits inlength, and may include one header flit and 4 data flits. In order toreduce idle latency, the headered flit may send data flit 1 twice. Thefirst time it is sent with only the data from the first 32 bytes and isreferred to as flit la. This allows the flit to be sent before thesecond 32 bytes of data is available from the memory devices. The resultis that the critical 32 bytes chunk of data arrives earlier at the hostfor the idle case. Flit 1 can then be sent a second time, this timecomplete. It will contain a repeat of the first 32 byte data portionplus the second 32 byte portion. Table 3 describes the five flits of aheadered packet:

TABLE 3 Headered Packet Flit Contents Header Packet Header Data flit 0Data from the first 32 bytes of the cache line Data flit 1a Remainder ofthe first 32 bytes only. Second 32-byte portion is reserved. The otherflit payload bits (RTID[6:3]) are identical to Data Flit 1. Data flit 1Complete Flit 1. First 32-byte portion is resent. Second 32-byte portionalso included. Data flit 2 Remainder of the second 32 bytes.In some implementations, headered MemData packets may not be cancelledand are to be followed by the data flits. A header is not to be sentuntil it known that the data can also be sent. The full cache line maynot be accumulated in the buffer before the header is sent.

In some systems, directory state of a cache line can be defined indirectory state bits included in the corresponding cache line itself. Inone example, the directory state bits can be included in the ECC bits ofthe cache line. Accordingly, in some implementations, an update to thedirectory state can involve rewriting the entire cache line (e.g.,because the ECC must be updated based on the directory state change).The cache line would not be rewritten if the directory state does notchange. For read and invalidate commands, the directory bits returned tothe host are the original directory information before the update. Allsubsequent reads would then return the new state, even if they occurbefore the data has been written back to memory, (i.e., the cache linewith the updated directory is globally visible).

Turning to FIG. 16, a simplified flow diagram 1600 is shown illustratingexample transactions involving a host 1305 and memory controller 1420.The transactions illustrated may be carried out, at least in part,between a host 1305 and buffer chip 1315 over a buffered memoryinterconnect, such as introduced above. In the example of FIG. 16, aprocessor associated with a local coherence agent 1605 (e.g., a homeagent) can attempt to proceed with a full cache line write of aparticular cache line in system memory (e.g., in far memory 1410). Thelocal agent 1605 can send a request to obtain exclusive ownership of thecache line with an InvItoE request 1610. In this example, the localagent 1605 has invalidated (I) state access to the line as it is localand not covered by the directory. Further, the InvItoE request 1610 canbe a defined request within a GPIO interconnect protocol, such as thatdescribed above. The InvItoE request 1610 can be received by the homeagent 1615. In response, the home agent 1615 can generate snoops to sendto other coherence agents (e.g., 1620). For instance, the home agent1615 can generate a snoop invalidate SnpinvItoE 1625 to send to one ormore remote coherence agents (e.g., 1620) to determine the cachestate(s) of the remote caches. The snoop invalidate 1625 can be sent asa result of the InvItoE request 1610 and can cause the copy of the cacheline at the receiving coherence agent 1620 to be invalidated. Snoopinvalidates can also cause any modified, or “dirty”, copies of the cacheline to be flushed to system memory from the cache holding the dirtycopy. In some cases, the flushing of a dirty cache line to near memorycan occur first to give the home agent 1615 an opportunity to determinehow to route the data.

Further, in response to receiving the InvItoE request 1605, the homeagent 1615 can send a request (e.g., defined by buffered memoryinterface) to a memory controller 1420 to accommodate the request of thecoherency agent 1605 to obtain an exclusive copy of the cache line. Inthis case, the request can be a memory invalidate request 1625(MemInvXtoI). In response to the MemInvXtoI request 1625, no read datais to be returned. The memory controller 1420 reads (at 1630) the cacheline in response to the MemInvXtoI request 1625 and if the cacheline isnot already in the invalid (“I”) state, the memory controller rewritesthe cache line such that its directory state is set to I. In a 2LMsystem, as in the example of FIG. 16, the memory controller can firstcheck corresponding near memory 1405 for the cache line that is thesubject of the MemInvXtoI request. In this example, a “miss” is returnedfor the requested near memory address (e.g., at 1635). Additionally, theother cache line found at the address can be a modified copy.Accordingly, the memory controller 1420 flush the modified other cacheline to far memory 1410 (through write 1640). The memory controller1420, in response to the miss, can then read (at 1645) the particularcache line (of the invalidate request 1625) from far memory 1410 inorder to overwrite (at 1650) the corresponding near memory address withthe particular cache line. The memory controller 1420 can then send acompletion (CmpU) 1655 in response to the memory invalidate request1625. The completion can be an unordered completion (CmpU) or an orderedcompletion (CmpO) that is to be ordered with forward responses. In thisexample, the unordered completion (CmpU) (e.g., 1655) can be used as noordering requirements in this case between the memory controller 1420and the home agent 1615. For completions between the home agent 1615 andlocal agent 1605, an ordered completion (e.g., 1660) can be used suchthat completion is ordered in front of any later forward messages to thesame cache line address. Receipt of the completion 1655 can indicatethat the directory state has been changed. The completion (CmpO) 1660sent to the requesting coherence agent 1605 indicates to the coherenceagent 1605 that an exclusive copy of the requested cache line has beengranted to the local coherence agent 1605 (e.g., transitioning thedirectory state from I→E). With exclusive access, the coherence agent1605 can then proceed with the intended full cache line write. The writecauses the line to transition to a modified state and a correspondingwrite back can be performed in response. For instance, the coherenceagent 1605, after the line is modified, can send a WbMtoI request 1665to cause the cache line in modified M state to be written back to nearmemory (at 1670) and invalidate the line in the cache and change thedirectory state to invalid (I).

In an alternate implementation, illustrated in the simplified flowdiagrams of 17A-17B, a special command can be defined to optimize thegranting of an exclusive copy of a cache line to a coherence agent inadvance of a write. For instance, a MemInvItoX command 1705 can bedefined to be sent in connection with a full cache line write. TheMemInvItoX command 1705 can be a memory invalidate command definedaccording to a buffered memory interconnect protocol and can pertaindirectly to 2LM system memory topologies. The MemInvItoX command 1705can be one of multiple memory invalidate commands defined according to abuffered memory interconnect protocol. Unlike other memory invalidatecommands (e.g., MemInvXtoI), the directory is not caused to be updated(i.e., to I) by the MemInvItoX command 1705 itself

The MemInvItoX command 1705 can be sent in response to a request toobtain exclusive ownership of the cache line with an InvItoE request1610. The home agent can snoop other coherence agents (at 1625) todetermine the directory state of the requested line (in response to therequest to obtain exclusive ownership). Further, the MemInvItoX command1705 can cause near memory 1405 to be read (at 1710) and checked for thepresence of a copy of the desired line (i.e., the line that is to beoverwritten). A hit can result in the directory state being returnedfrom the read 1710 as held in the near memory copy. For a miss, aconservative, artificial directory state result can be assumed for thenear memory copy (as the copy and actual directory state value areunavailable in near memory). For instance, an any (A) state can beassumed for a line in the event of a miss for the line in itscorresponding near memory address. The assumed directory state can beselected conservatively such that it does not introduce coherency issuesor errors resulting from an incorrect directory state assumption. Thiscan further allow for far memory reads to be saved solely for directorystate.

In a MemInvItoX transaction, if the corresponding near memory read 1710results in both a “miss” at the address of the near memory 1405, and theaddress has a copy of another line in a dirty state (as in the exampleof FIG. 17A), the MemInvItoX command 1705 can cause the data of theother line to be flushed, or written to far memory (at 1715), in advanceof the corresponding full line write. The completion (e.g., 1720, 1725)for the MemInvItoX request 1705 can be sent before any the modified datais finished being flushed 1715 to far memory 1410. Indeed, rather thanadditionally reading far memory 1410 to pull the requested cache linefrom far memory and then writing the near memory address with the dataof the desired cache line, a completion is sent to a MemInvItoX request1705 immediately following the check of whether the desired cache lineis found at its near memory address, and the new cache line data can bewritten to the near memory address (at 1740) (e.g., because thecoherency agent 1605 tracks the line as inclusive in Near Memory).Indeed, as the MemInvItoX request 1705 is sent in connection with arequest (e.g., 1730) to perform a full cache line write, retrieving thesoon-to-be outdated copy of the desired line from far memory in responseto a read miss and writing this soon-to-be overwritten data in nearmemory can be skipped in favor of preserving the constrained bandwidthof far memory. Further, in a MemInvItoX request 1705, the onlyinteraction with far memory 1410 can be the writing of any dirty data(e.g., 1715) of an alternate cache line in the event of a near memorymiss. The flushing 1715 of dirty data can be carried outcontemporaneously with the sending of the completion 1720 to theMemInvItoX request 1705. Accordingly, a completion response (e.g., 1720,1725) to a MemInvItoX request 1705 can grant an exclusive copy of theparticular cache line associated with the MemInvItoX request 1705 toallow a host to then proceed with the overwrite (at 1735, 1740) of thedata in the corresponding near memory address with the particular cacheline (e.g., initiated through a WbMtoI, or inclusive MemWr, request1735).

As set forth in the example of FIG. 17A, in response to a MemInvItoXrequest 1705, a memory controller reads near memory to determine if acopy of the desired cache line is present. A MemInvItoX request 1705does not cause directory bits to be updated in near memory regardless ofa hit or miss. In the event of a near memory miss, no far memory read isperformed and if the near memory address contains modified, or dirty,data, that data is written to far memory since a later write to adifferent address within this set may overwrite it. In the event theread 1710 yields a near memory hit for the desired cache line, no changeis made to either near or far memory (prior to the full cache linewrite). For instance, as shown in FIG. 17B, a MemInvItoX request 1705can prompt a read 1710 of a corresponding address in near memory 1405.In this example, the read 1710 yields a near memory hit and the “real”directory state is returned. The transaction proceeds as in FIG. 17Awith a completion (e.g., 1720, 1725). However, in the case of a nearmemory hit, no data is written from near memory to far memory, even whenthe copy of the data is Modified (as the near memory is about to beupdated). A similar flow (as shown in FIG. 17B) can result in cases whenthe MemInvItoX request 1705 yields a read 1710 that results in a missbut with “clean” data (e.g., in a conservative directory state, as thecopy of the other cache line in the near memory location matches that infar memory 1410).

In the example of FIG. 17B, the completion (e.g., CmpO 1725) to theInvItoE request 1730 grants exclusive ownership of the line to therequester, allowing for a full line write to be carried out (e.g., at1735, 1740). The full line write can be performed using a WbMtoI request1735, which causes the directory state to return to invalid (I) state(i.e., M→I) following the write 1740.

HPI and an associated buffered memory access protocol can beincorporated in any variety of computing devices and systems, includingmainframes, server systems, personal computers, mobile computers (suchas tablets, smartphones, personal digital systems, etc.), smartappliances, gaming or entertainment consoles and set top boxes, amongother examples. For instance, referring to FIG. 18, shown is a blockdiagram of a second system 1800 in accordance with an embodiment of thepresent invention. As shown in FIG. 18, multiprocessor system 1800 is apoint-to-point interconnect system, and includes a first processor 1870and a second processor 1880 coupled via a point-to-point interconnect1850. Each of processors 1870 and 1880 may be some version of aprocessor. In one embodiment, 1852 and 1854 are part of a serial,point-to-point coherent interconnect fabric, such as a high-performancearchitecture. As a result, the invention may be implemented within theQPI architecture.

While shown with only two processors 1870, 1880, it is to be understoodthat the scope of the present invention is not so limited. In otherembodiments, one or more additional processors may be present in a givenprocessor. Processors 1870 and 1880 are shown including integratedmemory controller units 1872 and 1882, respectively. Processor 1870 alsoincludes as part of its bus controller units point-to-point (P-P)interfaces 1876 and 1878; similarly, second processor 1880 includes P-Pinterfaces 1886 and 1888. Processors 1870, 1880 may exchange informationvia a point-to-point (P-P) interface 1850 using P-P interface circuits1878, 1888. As shown in FIG. 18, IMCs 1872 and 1882 couple theprocessors to respective memories, namely a memory 1832 and a memory1834, which may be portions of main memory locally attached to therespective processors.

Processors 1870, 1880 each exchange information with a chipset 1890 viaindividual P-P interfaces 1852, 1854 using point to point interfacecircuits 1876, 1894, 1886, 1898. Chipset 1890 also exchanges informationwith a high-performance graphics circuit 1838 via an interface circuit1892 along a high-performance graphics interconnect 1839.

A shared cache (not shown) may be included in either processor oroutside of both processors; yet connected with the processors via P-Pinterconnect, such that either or both processors' local cacheinformation may be stored in the shared cache if a processor is placedinto a low power mode.

Chipset 1890 may be coupled to a first bus 1816 via an interface 1896.In one embodiment, first bus 1816 may be a Peripheral ComponentInterconnect (PCI) bus, or a bus such as a PCI Express bus or anotherthird generation I/O interconnect bus, although the scope of the presentinvention is not so limited.

As shown in FIG. 18, various I/O devices 1814 are coupled to first bus1816, along with a bus bridge 1818 which couples first bus 1816 to asecond bus 1820. In one embodiment, second bus 1820 includes a low pincount (LPC) bus. Various devices are coupled to second bus 1820including, for example, a keyboard and/or mouse 1822, communicationdevices 1827 and a storage unit 1828 such as a disk drive or other massstorage device which often includes instructions/code and data 1830, inone embodiment. Further, an audio I/O 1824 is shown coupled to secondbus 1820. Note that other architectures are possible, where the includedcomponents and interconnect architectures vary. For example, instead ofthe point-to-point architecture of FIG. 18, a system may implement amulti-drop bus or other such architecture.

Referring now to FIG. 19, shown is a block diagram of an embodiment of amulticore processor. As shown in the embodiment of FIG. 19, processor1900 includes multiple domains. Specifically, a core domain 1930includes a plurality of cores 1930A-1930N, a graphics domain 1960includes one or more graphics engines having a media engine 1965, and asystem agent domain 1910.

In various embodiments, system agent domain 1910 handles power controlevents and power management, such that individual units of domains 1930and 1960 (e.g. cores and/or graphics engines) are independentlycontrollable to dynamically operate at an appropriate power mode/level(e.g. active, turbo, sleep, hibernate, deep sleep, or other AdvancedConfiguration Power Interface like state) in light of the activity (orinactivity) occurring in the given unit. Each of domains 1930 and 1960may operate at different voltage and/or power, and furthermore theindividual units within the domains each potentially operate at anindependent frequency and voltage. Note that while only shown with threedomains, understand the scope of the present invention is not limited inthis regard and additional domains may be present in other embodiments.

As shown, each core 1930 further includes low level caches in additionto various execution units and additional processing elements. Here, thevarious cores are coupled to each other and to a shared cache memorythat is formed of a plurality of units or slices of a last level cache(LLC) 1940A-1940N; these LLCs often include storage and cache controllerfunctionality and are shared amongst the cores, as well as potentiallyamong the graphics engine too.

As seen, a ring interconnect 1950 couples the cores together, andprovides interconnection between the core domain 1930, graphics domain1960 and system agent circuitry 1910, via a plurality of ring stops1952A-1952N, each at a coupling between a core and LLC slice. As seen inFIG. 19, interconnect 1950 is used to carry various information,including address information, data information, acknowledgementinformation, and snoop/invalid information. Although a ring interconnectis illustrated, any known on-die interconnect or fabric may be utilized.As an illustrative example, some of the fabrics discussed above (e.g.another on-die interconnect, On-chip System Fabric (OSF), an AdvancedMicrocontroller Bus Architecture (AMBA) interconnect, amulti-dimensional mesh fabric, or other known interconnect architecture)may be utilized in a similar fashion.

As further depicted, system agent domain 1910 includes display engine1912 which is to provide control of and an interface to an associateddisplay. System agent domain 1910 may include other units, such as: anintegrated memory controller 1920 that provides for an interface to asystem memory (e.g., a DRAM implemented with multiple DIMMs; coherencelogic 1922 to perform memory coherence operations. Multiple interfacesmay be present to enable interconnection between the processor and othercircuitry. For example, in one embodiment at least one direct mediainterface (DMI) 1916 interface is provided as well as one or more PCIe™interfaces 1914. The display engine and these interfaces typicallycouple to memory via a PCIe™ bridge 1918. Still further, to provide forcommunications between other agents, such as additional processors orother circuitry, one or more other interfaces may be provided.

While the present invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations as fall within thetrue spirit and scope of this present invention.

A design may go through various stages, from creation to simulation tofabrication. Data representing a design may represent the design in anumber of manners. First, as is useful in simulations, the hardware maybe represented using a hardware description language (HDL) or anotherfunctional description language. Additionally, a circuit level modelwith logic and/or transistor gates may be produced at some stages of thedesign process. Furthermore, most designs, at some stage, reach a levelof data representing the physical placement of various devices in thehardware model. In the case where conventional semiconductor fabricationtechniques are used, the data representing the hardware model may be thedata specifying the presence or absence of various features on differentmask layers for masks used to produce the integrated circuit. In someimplementations, such data may be stored in a database file format suchas Graphic Data System II (GDS II), Open Artwork System InterchangeStandard (OASIS), or similar format.

In some implementations, software based hardware models, and HDL andother functional description language objects can include registertransfer language (RTL) files, among other examples. Such objects can bemachine-parsable such that a design tool can accept the HDL object (ormodel), parse the HDL object for attributes of the described hardware,and determine a physical circuit and/or on-chip layout from the object.The output of the design tool can be used to manufacture the physicaldevice. For instance, a design tool can determine configurations ofvarious hardware and/or firmware elements from the HDL object, such asbus widths, registers (including sizes and types), memory blocks,physical link paths, fabric topologies, among other attributes thatwould be implemented in order to realize the system modeled in the HDLobject. Design tools can include tools for determining the topology andfabric configurations of system on chip (SoC) and other hardware device.In some instances, the HDL object can be used as the basis fordeveloping models and design files that can be used by manufacturingequipment to manufacture the described hardware. Indeed, an HDL objectitself can be provided as an input to manufacturing system software tocause the described hardware.

In any representation of the design, the data may be stored in any formof a machine readable medium. A memory or a magnetic or optical storagesuch as a disc may be the machine readable medium to store informationtransmitted via optical or electrical wave modulated or otherwisegenerated to transmit such information. When an electrical carrier waveindicating or carrying the code or design is transmitted, to the extentthat copying, buffering, or re-transmission of the electrical signal isperformed, a new copy is made. Thus, a communication provider or anetwork provider may store on a tangible, machine-readable medium, atleast temporarily, an article, such as information encoded into acarrier wave, embodying techniques of embodiments of the presentdisclosure.

A module as used herein refers to any combination of hardware, software,and/or firmware. As an example, a module includes hardware, such as amicro-controller, associated with a non-transitory medium to store codeadapted to be executed by the micro-controller. Therefore, reference toa module, in one embodiment, refers to the hardware, which isspecifically configured to recognize and/or execute the code to be heldon a non-transitory medium. Furthermore, in another embodiment, use of amodule refers to the non-transitory medium including the code, which isspecifically adapted to be executed by the microcontroller to performpredetermined operations. And as can be inferred, in yet anotherembodiment, the term module (in this example) may refer to thecombination of the microcontroller and the non-transitory medium. Oftenmodule boundaries that are illustrated as separate commonly vary andpotentially overlap. For example, a first and a second module may sharehardware, software, firmware, or a combination thereof, whilepotentially retaining some independent hardware, software, or firmware.In one embodiment, use of the term logic includes hardware, such astransistors, registers, or other hardware, such as programmable logicdevices.

Use of the phrase ‘configured to,’ in one embodiment, refers toarranging, putting together, manufacturing, offering to sell, importingand/or designing an apparatus, hardware, logic, or element to perform adesignated or determined task. In this example, an apparatus or elementthereof that is not operating is still ‘configured to’ perform adesignated task if it is designed, coupled, and/or interconnected toperform said designated task. As a purely illustrative example, a logicgate may provide a 0 or a 1 during operation. But a logic gate‘configured to’ provide an enable signal to a clock does not includeevery potential logic gate that may provide a 1 or 0. Instead, the logicgate is one coupled in some manner that during operation the 1 or 0output is to enable the clock. Note once again that use of the term‘configured to’ does not require operation, but instead focus on thelatent state of an apparatus, hardware, and/or element, where in thelatent state the apparatus, hardware, and/or element is designed toperform a particular task when the apparatus, hardware, and/or elementis operating.

Furthermore, use of the phrases ‘to,’ ‘capable of/to,’ and or ‘operableto,’ in one embodiment, refers to some apparatus, logic, hardware,and/or element designed in such a way to enable use of the apparatus,logic, hardware, and/or element in a specified manner. Note as abovethat use of to, capable to, or operable to, in one embodiment, refers tothe latent state of an apparatus, logic, hardware, and/or element, wherethe apparatus, logic, hardware, and/or element is not operating but isdesigned in such a manner to enable use of an apparatus in a specifiedmanner.

A value, as used herein, includes any known representation of a number,a state, a logical state, or a binary logical state. Often, the use oflogic levels, logic values, or logical values is also referred to as 1'sand 0's, which simply represents binary logic states. For example, a 1refers to a high logic level and 0 refers to a low logic level. In oneembodiment, a storage cell, such as a transistor or flash cell, may becapable of holding a single logical value or multiple logical values.However, other representations of values in computer systems have beenused. For example the decimal number ten may also be represented as abinary value of 1010 and a hexadecimal letter A. Therefore, a valueincludes any representation of information capable of being held in acomputer system.

Moreover, states may be represented by values or portions of values. Asan example, a first value, such as a logical one, may represent adefault or initial state, while a second value, such as a logical zero,may represent a non-default state. In addition, the terms reset and set,in one embodiment, refer to a default and an updated value or state,respectively. For example, a default value potentially includes a highlogical value, i.e. reset, while an updated value potentially includes alow logical value, i.e. set. Note that any combination of values may beutilized to represent any number of states.

The embodiments of methods, hardware, software, firmware or code setforth above may be implemented via instructions or code stored on amachine-accessible, machine readable, computer accessible, or computerreadable medium which are executable by a processing element. Anon-transitory machine-accessible/readable medium includes any mechanismthat provides (i.e., stores and/or transmits) information in a formreadable by a machine, such as a computer or electronic system. Forexample, a non-transitory machine-accessible medium includesrandom-access memory (RAM), such as static RAM (SRAM) or dynamic RAM(DRAM); ROM; magnetic or optical storage medium; flash memory devices;electrical storage devices; optical storage devices; acoustical storagedevices; other form of storage devices for holding information receivedfrom transitory (propagated) signals (e.g., carrier waves, infraredsignals, digital signals); etc., which are to be distinguished from thenon-transitory mediums that may receive information there from.

Instructions used to program logic to perform embodiments of theinvention may be stored within a memory in the system, such as DRAM,cache, flash memory, or other storage. Furthermore, the instructions canbe distributed via a network or by way of other computer readable media.Thus a machine-readable medium may include any mechanism for storing ortransmitting information in a form readable by a machine (e.g., acomputer), but is not limited to, floppy diskettes, optical disks,Compact Disc, Read-Only Memory (CD-ROMs), and magneto-optical disks,Read-Only Memory (ROMs), Random Access Memory (RAM), ErasableProgrammable Read-Only Memory (EPROM), Electrically ErasableProgrammable Read-Only Memory (EEPROM), magnetic or optical cards, flashmemory, or a tangible, machine-readable storage used in the transmissionof information over the Internet via electrical, optical, acoustical orother forms of propagated signals (e.g., carrier waves, infraredsignals, digital signals, etc.). Accordingly, the computer-readablemedium includes any type of tangible machine-readable medium suitablefor storing or transmitting electronic instructions or information in aform readable by a machine (e.g., a computer).

The following examples pertain to embodiments in accordance with thisSpecification. One or more embodiments may provide an apparatus, asystem, a machine readable storage, a machine readable medium, hardware-and/or software-based logic (e.g., implemented in connection with amemory controller), and a method to receive a particular memoryinvalidation request that references a particular line of far memory ina two level system memory topology with the far memory and near memory,identify a particular address of the near memory corresponding to theparticular line, read the particular address of the near memory todetermine whether a copy of the line is in the near memory, flush dataof the particular address to the far memory if the data includes a copyof another line of the far memory and the copy of the other lineincludes modified data, and send a completion for the particular memoryinvalidation request to indicate that a coherence agent is granted anexclusive copy of the particular line. The exclusive copy of theparticular line is to be modified to generate a modified version of theparticular line and the particular address of the near memory is to beoverwritten with the modified version of the particular line.

In one example, the completion is to be sent prior to completion of anyreads and writes to far memory responsive to the particular memoryinvalidation requests.

In one example, a memory controller is to identify from the read that acopy of a line in far memory other than the particular line is stored inthe particular address of the near memory, identify that the copy of theline other than the particular line includes modified data, andoverwrite the line in far memory to include the modified data.

In one example, a copy of the particular line is not to be written tothe particular address prior to the particular address of near memorybeing overwritten with the modified version of the particular line.

In one example, the memory controller is further to receive a writerequest to write the modified version of the particular line to theparticular address of the near memory, and cause the particular addressof the near memory to be overwritten with the modified version of theparticular line.

In one example, the write request is received from a coherence agent.

In one example, the write request is to cause directory state of theparticular line to transition from modified to invalid.

In one example, directory state of the copy of the line in theparticular address is to be identified in the read.

In one example, if another line of far memory is identified in the read,directory state of the particular line is to be assumed to be an any (A)state.

In one example, the memory controller is included in a buffered memorychip.

In one example, the particular memory invalidation request is to bereceived over a buffered memory interface.

In one example, the buffered memory interface is separate from a generalpurpose input/output (GPIO) interface and the buffered memory interfaceincludes a physical layer based on a physical layer of the generalpurpose input/output (GPIO) interface.

In one example, the particular memory invalidation request is one of aplurality of memory invalidation requests.

In one example, the particular memory invalidation request correspondsto a request indicating an upcoming full line write of the particularline.

In one example, no writes or reads of far memory are to be performed inresponse to the particular memory invalidation request when the readreveals a clean copy of a line other than the particular line in theparticular address of near memory.

In one example, directory state of the particular line is not changeddirectly in response to the particular memory invalidation request.

One or more embodiments may provide a system that includes a bufferchip, two layer memory to be accessed through the buffer chip thatincludes near memory and far memory, and a processor block coupled tothe buffer chip by a memory access link. The buffer chip can include amemory controller to receive a particular memory invalidation requestreferencing a particular line of far memory, identify a particularaddress of near memory corresponding to the particular line, read theparticular address of near memory to determine whether a copy of theline is in the near memory, flush data of the particular address to thefar memory if the data includes a copy of another line of far memory andthe copy of the other line includes modified data, and send a completionfor the particular memory invalidation request to indicate that acoherence agent is granted an exclusive copy of the particular line. Theexclusive copy of the particular line is to be modified to generate amodified version of the particular line and the particular address ofnear memory is to be overwritten with the modified version of theparticular line.

In one example, the system also includes a home agent.

In one example, the home agent is to send a snoop to one or more othercoherence agents based on the particular memory invalidation request.

In one example, the snoop includes a snoop invalidate.

In one example, the processor block is to interface with one or moreother devices over a general purpose input/output (GPIO) interconnectlink, the memory access link is different from the GPIO interconnectlink, and a physical layer of the memory access link is based on aphysical layer of the GPIO interconnect link.

In one example, the memory access link includes a buffered memory accesslink and the particular memory invalidation request is to be receivedover the buffered memory access link.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

In the foregoing specification, a detailed description has been givenwith reference to specific exemplary embodiments. It will, however, beevident that various modifications and changes may be made theretowithout departing from the broader spirit and scope of the invention asset forth in the appended claims. The specification and drawings are,accordingly, to be regarded in an illustrative sense rather than arestrictive sense. Furthermore, the foregoing use of embodiment andother exemplarily language does not necessarily refer to the sameembodiment or the same example, but may refer to different and distinctembodiments, as well as potentially the same embodiment.

What is claimed is:
 1. An apparatus comprising: a memory controller toaccess a two level memory, wherein the two level memory comprises a nearmemory and a far memory, the near memory is to serve as a cache for thefar memory, and the memory controller is to: receive a particular memoryinvalidation request, wherein the particular memory invalidation requestis to reference a particular line of far memory; identify a particularaddress of near memory associated with the particular line; read theparticular address of near memory to determine whether a copy of theline is in the near memory, wherein the memory controller is to flushdata of the particular address to the far memory if the data identifiedin the read comprises a copy of another line of far memory and the copyof the other line comprises modified data; and send a completion for theparticular memory invalidation request to indicate that a coherenceagent is granted an exclusive copy of the particular line, wherein theexclusive copy of the particular line is to be modified to generate amodified version of the particular line and the particular address ofnear memory is to be overwritten with the modified version of theparticular line.
 2. The apparatus of claim 1, wherein the completion isto be sent prior to completion of any reads and writes to far memoryresponsive to the particular memory invalidation requests.
 3. Theapparatus of claim 1, wherein the memory controller is further to:identify from the read that a copy of a line in far memory other thanthe particular line is stored in the particular address of the nearmemory; identify that the copy of the line other than the particularline comprises modified data; and overwrite the line in far memory toinclude the modified data.
 4. The apparatus of claim 3, wherein a copyof the particular line is not to be written to the particular addressprior to the particular address of near memory being overwritten withthe modified version of the particular line.
 5. The apparatus of claim1, wherein the memory controller is further to: receive a write requestto write the modified version of the particular line to the particularaddress of the near memory; and cause the particular address of the nearmemory to be overwritten with the modified version of the particularline.
 6. The apparatus of claim 5, wherein the write request is receivedfrom a coherence agent.
 7. The apparatus of claim 5, wherein the writerequest is to cause directory state of the particular line to transitionfrom modified to invalid.
 8. The apparatus of claim 1, wherein directorystate of the copy of the line in the particular address is to beidentified in the read.
 9. The apparatus of claim 8, wherein if anotherline of far memory is identified in the read, directory state of theparticular line is to be assumed to be an any (A) state.
 10. Theapparatus of claim 1, wherein the memory controller is included in abuffered memory chip.
 11. The apparatus of claim 10, wherein theparticular memory invalidation request is to be received over a bufferedmemory interface.
 12. The apparatus of claim 11, wherein the bufferedmemory interface is separate from a general purpose input/output (GPIO)interface and the buffered memory interface comprises a physical layerbased on a physical layer of the general purpose input/output (GPIO)interface.
 13. The apparatus of claim 1, wherein the particular memoryinvalidation request is one of a plurality of memory invalidationrequests.
 14. The apparatus of claim 13, wherein the particular memoryinvalidation request corresponds to a request indicating an upcomingfull line write of the particular line.
 15. The apparatus of claim 1,wherein no writes or reads of far memory are to be performed in responseto the particular memory invalidation request when the read reveals aclean copy of a line other than the particular line in the particularaddress of near memory.
 16. The apparatus of claim 1, wherein directorystate of the particular line is not changed directly in response to theparticular memory invalidation request.
 17. A method comprising:receiving a particular memory invalidation request, wherein theparticular memory invalidation request references a particular line offar memory in a two level system memory topology, and the two levelsystem memory topology comprises the far memory and near memory;identifying a particular address of the near memory corresponding to theparticular line; reading the particular address of the near memory todetermine whether a copy of the line is in the near memory, wherein dataof the particular address is to be flushed to the far memory if the datacomprises a copy of another line of the far memory and the copy of theother line comprises modified data; and sending a completion for theparticular memory invalidation request to indicate that a coherenceagent is granted an exclusive copy of the particular line, wherein theexclusive copy of the particular line is to be modified to generate amodified version of the particular line and the particular address ofthe near memory is to be overwritten with the modified version of theparticular line.
 18. At least one machine accessible storage mediumhaving code stored thereon, the code when executed on a machine, causesthe machine to: receive a particular memory invalidation request,wherein the particular memory invalidation request references aparticular line of far memory in a two level system memory topology, andthe two level system memory topology comprises the farm memory and nearmemory; identify a particular address of the near memory correspondingto the particular line; read the particular address of the near memoryto determine whether a copy of the line is in the near memory, whereinthe memory controller is to flush data of the particular address to thefar memory if the data comprises a copy of another line of the farmemory and the copy of the other line comprises modified data; and senda completion for the particular memory invalidation request to indicatethat a coherence agent is granted an exclusive copy of the particularline, wherein the exclusive copy of the particular line is to bemodified to generate a modified version of the particular line and theparticular address of the near memory is to be overwritten with themodified version of the particular line.
 19. A system comprising: abuffer chip; two layer memory to be accessed through the buffer chip,wherein the two layer memory comprises near memory and far memory; and aprocessor block coupled to the buffer chip by a memory access link;wherein the buffer chip comprises a memory controller to: receive aparticular memory invalidation request, wherein the particular memoryinvalidation request references a particular line of far memory;identify a particular address of near memory corresponding to theparticular line; read the particular address of near memory to determinewhether a copy of the line is in the near memory, wherein the memorycontroller is to flush data of the particular address to the far memoryif the data comprises a copy of another line of far memory and the copyof the other line comprises modified data; and send a completion for theparticular memory invalidation request to indicate that a coherenceagent is granted an exclusive copy of the particular line, wherein theexclusive copy of the particular line is to be modified to generate amodified version of the particular line and the particular address ofnear memory is to be overwritten with the modified version of theparticular line.
 20. The system of claim 19, further comprising a homeagent.
 21. The system of claim 20, wherein the home agent is to send asnoop to one or more other coherence agents based on the particularmemory invalidation request.
 22. The system of claim 21, wherein thesnoop comprises a snoop invalidate.
 23. The system of claim 19, whereinthe processor block is to interface with one or more other devices overa general purpose input/output (GPIO) interconnect link, the memoryaccess link is different from the GPIO interconnect link, and a physicallayer of the memory access link is based on a physical layer of the GPIOinterconnect link.
 24. The system of claim 23, wherein the memory accesslink comprises a buffered memory access link and the particular memoryinvalidation request is to be received over the buffered memory accesslink.